Why do all the super heros wear SPANDEX.  

Nothing's more embarrassing than splitting your pants in the middle of kicking a villains arse.

If you've ever tried any super hero activity yourself you'd know that the crotches in most modern clothing would not allow for all that jumping and kicking without splitting in the first 5 minutes.

Yes that's right about the stretchy bits.
Only SPANDEX will give and stretch when a Super Hero stretches their huge muscles.

I think only the Hulk had to go without a suit.

They tried to get him to wear a lovely pink and mauve creation but he just went all green.

Thats a good question. I suppose they want to show off their muscles. There's really no other reason. They look stupider in their costumes, but they are easily IDed as a superhero.

It is next to impossible to destory spandex, which why surfers wear it--if superheroes existed, they would be using it because of the beating that it can take. Also because it is body hugging, it is only thing that you can make a costume of and concealed under your street clothes. If you have a secret identity, spandex is your friend.