Why couldn't Harry see Thestrals at the end of Goblet of Fire after the senseless and shocking death of Cedric Diggory?
At the end of Goblet of Fire, the carriages still appeared horseless to Harry:
Hermione turned away, smiling at the horseless carriages which were now trundling towards them up the drive, as Krum, looking surprised, but gratified, signed a fragment of parchment for Ron.
Goblet of Fire - Page 629 - British Hardcover - Adult Edition
At the beginning of Order of the Phoenix, Harry sees the Thestrals for the first time:
Here stood the hundred or so horseless stagecoaches that always took the students above first year up to the castle. Harry glanced quickly at them, turned away to keep a lookout for Ron and Hermione, then did a double-take.
The coaches were no longer horseless. There were creatures standing between the carriage shafts.
Order of the Phoenix - Page 178 - British Hardcover - Adult Edition
We know that only those who have seen death can see Thestrals:
‘Don’ worry, it won’ hurt yeh,’ said Hagrid patiently. ‘Righ’, now, who can tell me why some o’ yeh can see ’em an’ some can’t?’
Hermione raised her hand.
‘Go on then,’ said Hagrid, beaming at her.
‘The only people who can see Thestrals,’ she said, ‘are people who have seen death.’
‘Tha’s exactly right,’ said Hagrid solemnly [...]
Order of the Phoenix - Page 394 - British Hardcover - Adult Edition
A month went by between Cedric's death and the end of the spring term -- is this enough time for Harry to have truly internalized Cedric's death? Goblet of Fire indicates that Harry was in shock following Cedric's murder, but was able to relate the story of Cedric's death to Mr. and Mrs. Diggory.
When he looked back, even a month later, Harry found he had few memories of the following days. It was as though he had been through too much to take in any more. The recollections he did have were very painful. The worst, perhaps, was the meeting with the Diggorys that took place the following morning.
‘He suffered very little, then,’ she said, when Harry had told her how Cedric had died.'
Goblet of Fire - Page 621 - British Hardcover - Adult Edition
So, then, why couldn't Harry see Thestrals at the end of Goblet of Fire?