I think it's because usually the punchline is too long
He solved for y! Thought this one up myself and thought it was post worthy
The other blonde says, "Well duh! You can't see Florida from here."
A school bus full of black people (Sorry for racism, it is just a joke, nothing personal)
Because they're funny on many levels.
What do you call a nun that sleep-walks a) A Roamin' Catholic b) An unconscious habit
THE PUNCHLINE OF THIS JOKE HAS BEEN CENSORED BY THE GOVERNMENT OF THE PEOPLES REPUBLIC OF CHINA, PLEASE REMAIN CALM WHILE WE WILL DEAL WITH THE OP IN A CIVIL MANNER.
The punchlines are too long.