Moral reasons
Droids fresh from the factory generally aren’t particularly sapient. Their systems are sufficiently complex, however, that a droid that operates for a while without memory wipes may become sentient:
The droids we see bias toward the sentient, though. It is probably in part the relative lack of sentience of most droids that leads even those that are obviously thinking beings to be considered legally property in many Star Wars legal systems.
Now, some people in real life might hesitate to erase the memory of a sapient entity, be it biological or mechanical. Despite the general prevalence of droids in Star Wars, and their apparent legal status as property, it’s not unreasonable to suppose that many might share this opinion.
Indeed, while droids are definitely considered property throughout much of the Galaxy at the time of the Republic, this is by no means a universal opinion. Some planets and individuals treat sentient droids as, well, sentient:
Padmé then had extended her hand and graciously invited C-3PO to join
her staff, because on Naboo, high-functioning droids were respected as
thinking beings, and 3PO had been so flustered at being treated like a
sentient creature that he’d been barely able to speak, beyond
muttering something about hoping he might make himself useful, because
after all he was “fluent in over six million forms of communication.”
Revenge of the Sith
As might come as no surprise, morally dubious actors such as the Galactic Empire and the corrupt Republic tend to treat droids as slaves or property, even if sentient, and try to prevent them from developing an personality, whereas “good guys” like Naboo and the Alliance tend to treat droids as thinking beings.
According to Rayne Roberts of Lucasfilm:
Now as you may know, the Empire relies on astromech droids to maintain
its machinery. But unlike the Rebel Alliance, Imperial technicians do
not grant their droids independence, and subject them to frequent
memory wipes to keep them subservient.
So let’s consider the people who could reasonably have "upgraded" C-3PO’s personality.
Anakin Skywalker
To start with, Anakin built C-3PO, so he’s the one who gave him his self-consciousness in the first place. What’s more, he seemed to think of C-3PO as basically human and to want him to be free:
Anakin had only laughed. What use is a protocol droid to a Jedi? Even
one as upgraded as 3PO—Anakin had packed his creation with so many
extra circuits and subprograms and heuristic algorithms that the droid
was practically human.
“I’m not giving him to you,” he’d told her. “He’s not even really mine
to give; when I built him, I was a slave, and everything I did
belonged to Watto. Cliegg Lars bought him along with my mother; Owen
gave him back to me, but I’m a Jedi. I have renounced possessions. I
guess that means he’s free now. What I’m really doing is asking you to
look after him for me.”
“Look after him?”
“Yes. Maybe even give him a job. He’s a little fussy,” he’d admitted,
“and maybe I shouldn’t have given him quite so much
self-consciousness—he’s a worrier—but he’s very smart, and he might be
a real help to a big-time diplomat … like, say, a Senator from Naboo?”
Revenge of the Sith
He also viewed C-3PO as a friend:
“I didn’t have many friends when I was a kid,” he’d told her, “so I
built one.”
And C-3P0 had shuffled in behind him, gleaming as though he’d been
plated with solid gold.
Revenge of the Sith
He’d be a pretty poor candidate for changing C-3PO’s personality.
Padmé Amidala
As indicated in a preceding quote, she considers thinking droids to be the equivalent of organic sentients. It might even be illegal on Naboo to wipe the memory of a sentient droid, if they really do treat them as “thinking beings.” She’d arguably be even less likely to mess with C-3PO’s mind than Anakin.
Bail Organa
Well, he certainly had no hesitation wiping C-3PO’s memory, due to believing that he might be a security threat:
“How lovely!” C-3PO exclaimed. “His daughter is the child of Master
Anakin and Senator Amidala,” he explained to R2-D2. “I can hardly wait
to tell her all about her parents! I’m sure she will be very proud—”
“Oh, and the protocol droid?” Senator Organa said thoughtfully. “Have
its mind wiped.”
The captain saluted.
“Oh,” said C-3PO. “Oh, dear.”
Revenge of the Sith
On the other hand, he had no desire to deactivate R2-D2, despite believing him to be "too independent for a droid":
“How did you get in here?” Bail said the first thing that came into
his head.
“Artoo opened the hatch for me as soon as he got on board,” Ahsoka
said.
“I should have him deactivated,” Bail said with no real heat in his
voice. “He is far too independent for a droid.”
Ahsoka
While he does seem to hold prejudice against sentient droids, seeming not to consider them the equivalent of organics, he appears to consider them to have some rights or feelings that he should respect. He mainly wiped C-3PO’s memory because he was afraid that he would tell someone about his daughter. I think he would hesitate to go further than he considered necessary, for basically the same reason he refused to deactivate R2-D2.
Luke Skywalker
Luke has a close relationship with R2-D2, and seems to respect him as a thinking entity. He seems to respect him as a friend, and confides in him constantly:
Luke steps away from the repair work without ever really seeing it.
“I can’t do it, Artoo. I can’t go on alone.”
Beware the Power of the Dark Side!
And he clearly views him as a being capable of making his own decisions, and entitled to do so:
“Master Luke!” the protocol droid cried. “Is it true Artoo will be
accompanying you?”
“If that’s all right with Artoo,” Luke said, grinning. The little
droid’s head spun around as he squealed, lights flashing. “I’ll take
that as a yes.”
The Princess, the Scoundrel, and the Farm Boy
It seems unlikely that he would not extend the same courtesy to C-3PO. As such, he probably would have an aversion to the idea of altering C-3PO’s personality.
Leia Organa
Similar arguments apply. Like Luke, she generally seemed to see C-3PO and R2-D2 as people.
All in all, each of C-3PO’s “owners” appears to have had sufficient respect for the rights of thinking droids to make it unlikely that they would have changed C-3PO’s personality.
