Mainly because the common practice was to not allow a burial. The Crucifixion was suppose to humiliate the person completely. Doing so, they would allow the body to remain while wild animals devoured them, adding to the humiliation.
Unlike other convicts, the posting over His head had a political/religious slant.
The Jewish authorities had serious objection.
If the Romans execute any man standing forth as 'king of the Jews'...
or any man standing forth as a prophet....
Then all hope of ever coming out from under Rome is gone.
In the face of Jewish authority...Pilate refrained...
'I have written what I have written.'
However, when approached and the request made for the body, the Roman authorities allowed the body quickly removed.
Across the immediate country side, Jesus had been known to draw large crowds.
What if news of the execution be allowed to spread over several days?
BTW...He was nailed to hasten His death.
Most convicts were simply tied the cross.
Death followed by thirst or exposure.
Jesus was nailed...then lanced....that He not hang for the usual three days.