I doubt there's any single reason, but if pressed for a one-sentence summary I'd say that, on the balance, Malfoy's Hogwarts experience wasn't actually very positive. At the very least, it wouldn't have felt that way to him.
His last two years were quite difficult
His sixth year was an intensely stressful experience, which at one point led him to break down in tears:
"No one can help me," said Malfoy. His whole body was shaking. "I can't do it... I can't... It won't work... and unless I do it soon... he says he'll kill me..."
And Harry realized, with a shock so huge it seemed to root him to the spot, that Malfoy was crying — actually crying — tears streaming down his pale face into the grimy basin.
Half-Blood Prince Chapter 24: "Sectumsempra"
Psychologically speaking, people tend to weigh negative emotions more heavily than positive ones, and Draco had a very negative sixth year.
Although we don't get as much insight into his mental state, I would also suggest that his seventh year probably wasn't very pleasant either. Not only has he lost much of his former status, but Rowling is quite clear that he's beginning to question his loyalty to Voldemort, as she writes on his Pottermore page:
Draco's changed, yet still conflicted, personality revealed itself in his actions during the remainder of the war between Voldemort and those who were trying to stop him.
Pottermore Draco Malfoy
Although I doubt his seventh year was quite as stressful as his sixth, between his growing conscience and diminished status, I'm sure it was a bit of a rude awakening for him.
By the time of The Cursed Child, he's a changed man
I touched on this a little in the last section, but it bears expansion: by the time of Scorpius' birth, Draco has undergone some considerable personal growth since his Hogwarts days. In his first year, he was the picture of pure-blood arrogance; by the time he turned eighteen, he had gone through a rather traumatic re-education.
It wouldn't surprise me if, in his adulthood, he resented the kind of person he had been in those days; I think this is something a lot of people go through to some extent (especially looking back on your teenaged years), but looking back twenty years later on your past as a militant racist must be something else entirely.
It's not hard for that kind of resentment to colour even otherwise-positive memories.
Hogwarts really didn't live up to his expectations
Reading his Pottermore page, it's clear that Malfoy had very specific expectations about what his Hogwarts experience would be like; a particlarly notable line is:
From the time when he could talk, it was made clear to him that he was triply special: firstly as a wizard, secondly as a pure-blood, and thirdly as a member of the Malfoy family.
Pottermore Draco Malfoy
So Malfoy clearly went into his first year expecting to be Crown Prince of Hogwarts, something which obviously didn't happen; again from Pottermore:
Harry was unquestionably the most talked-about and admired person at school, and this naturally jarred with a boy who had been brought up to believe that he occupied an almost royal position within the wizarding community. What was more, Harry was most talented at flying, the one skill at which Malfoy had been confident he would outshine all the other first-years.
Pottermore Draco Malfoy
He was surpassed by Harry Potter, his greatest enemy, at the two areas he thought he should excel at: popularity and flying. In fact, Harry was so much better than him at flying that he was permitted to own a broom (against the rules) and join the Quidditch team (highly unusual).
Harry, his enemy, had everything he wanted
Again, this is a point that's worth expanding on, because Harry is basically everything Malfoy wanted to be, and thought he was: popular, talented, and special. As the Designated Hero, Harry basically succeeds at everything he does; he has hardships, sure, but it all ultimately works out for him.
Malfoy may have had a lot going for him, but Harry had all of the things he wanted; he's known by everyone, well-regarded (for the most part) by staff and students alike, and (from the perspective of an outsider) he basically always gets what he wants. What's more (as pointed out by NKCampbell in a comment on the question, and by CassieD in an answer) he has actual friends, something Malfoy does not and which he greatly desires.
Finally, as discussed on that Pottermore page I'm so fond of linking to, even the Death Eaters see Harry as more important than Malfoy:
Much as the Death Eaters disliked Harry as an obstacle and as a symbol, he was discussed seriously as an adversary, whereas Draco was still relegated to the status of schoolboy by Death Eaters who met at his parents' house. Though they were on opposing sides of the gathering battle, Draco felt envious of Harry’s status.
Pottermore Draco Malfoy
All of his efforts to overcome Harry, either by diminishing his status or by building his (Malfoy's, that is) up, ultimately failed; which brings me to my next point:
He failed at just about everything he tried
Of course, he frequently lost against Harry on the Quidditch pitch; although Slytherin usually beat the other Houses, they never beat Gryffindor in any year Harry played, and Gryffindor won the Quidditch Cup every year they possibly could have (excluding Chamber of Secrets, where the tournament was cancelled mid-season, Goblet of Fire, where it was supplanted by the Triwizard Tournament, and Deathly Hallows, where it may not have been held at all).
Despite knowing for a fact that Harry et al. were transporting an illegal dragon after curfew, he still gets caught and punished for a relatively minor infraction:
"Detention!" [McGonagall] shouted. "And twenty points from Slytherin! Wandering around in the middle of the night, how dare you -"
"You don't understand, Professor. Harry Potter's coming - he's got a dragon!"
Harry Potter and the Philosopher's Stone Chapter 14: "Norbert the Norwegian Ridgeback"
Despite breaking an untold number of rules, Harry et al. were rewarded by Dumbledore at the end of Philosopher's Stone, ultimately winning the House Cup from under Slytherin's nose:
Harry, still cheering, nudged Ron in the ribs and pointed at Malfoy, who couldn't have looked more stunned and horrified if he’d just had the Body-Bind Curse put on him.
Harry Potter and the Philosopher's Stone Chapter 17: "The Man With Two Faces"
In second year, despite convincing his father to spend what was probably a tidy sum on seven top-of-the-line brooms, he still lost, which must have been a massive confidence drain; I'm sure Lucius wasn't pleased with him either
- In fourth year, attempting to get back at Harry for insulting his mother resulted in his being very publicly humiliated by a professor, and getting bailed out by the Head of Gryffindor
- Also in fourth year, despite Malfoy's smear campaign Harry ended up winning the Tri-Wizard Tournament
In fifth year he toadied up to Umbridge, which of course ended disatrously:
[A]t the very moment of triumph, when Draco had cornered Harry and his comrades, and when it seemed that Harry must be expelled by Umbridge, Harry slipped through his fingers. Worse still, Harry managed to thwart Lucius Malfoy's attempt to kill him, and Draco’s father was captured and sent to Azkaban.
Pottermore Draco Malfoy
Also in his fifth year, Ron ended up being the Quidditch hero despite Malfoy's "Weasley Is Our King" campaign
He also failed quite effectively at things that weren't directly targeted at Harry, for example:
- The Heir of Slytherin debacle in second-year, which Draco was thrilled about at the time, ended up being resolved with no harm done, except for a substantial blow to his father's reputation. To say nothing of what must have happened once Voldemort found out.
In sixth year, aside from the tremendous stress, he was basically ignored by Slughorn, a man with a reputation for recognizing talent and success:
"Sir, I think you knew my grandfather, Abraxas Malfoy?" Harry looked up; Slughorn was just passing the Slytherin table.
"Yes," said Slughorn, without looking at Malfoy, "I was sorry to hear he had died, although of course it wasn't unexpected, dragon pox at his age..."
And he walked away. Harry bent back over his cauldron, smirking. He could tell that Malfoy had expected to be treated like Harry or Zabini; perhaps even hoped for some preferential treatment of the type he had learned to expect from Snape.
Half-Blood Prince Chapter 9: "The Half-Blood Prince"
Quite a blow for a young narcissist.
And, of course, he was constantly upstaged by Hermione academically, much to the displeasure of his father:
"I hope my son will amount to more than a thief or a plunderer, Borgin," said Mr. Malfoy coldly, and Mr. Borgin said quickly, "No offense, sir, no offense meant -"
"Though if his grades don't pick up,” said Mr. Malfoy, more coldly still, "that may indeed be all he is fit for -"
"The teachers all have favorites, that Hermione Granger -"
"I would have thought you'd be ashamed that a girl of no wizard family beat you in every exam," snapped Mr. Malfoy.
Harry Potter and the Chamber of Secrets Chapter 4: "At Flourish and Blotts"
Although we're not given a lot of insight into how he feels about that, I have to imagine it was a bit like how a racist must have felt watching Jesse Owens.
A lot of these sound very petty, and they are; however, I can say from experience that a hundred tiny annoyances build up over time, until optimism and enjoyment of your new school turns into resentment and misery.