Why Do Divers Wear Speedos?

It's because they allow you to be more flexible since they don't constrict the ability to move your legs as much as jammers do and they are more streamlined than board shorts.

Why do divers wear such small bathing suits?

So why are the divers' bathing suits so small? Aerodynamics and comfort, basically. As a male diver, you will spend a lot of time twisting around in midair, and you don't want your junk to fall out, or to get tangled in your suit while you're somersaulting.

Why do Olympic swimmers not wear speedos?

Fearing they gave swimmers a disproportionate advantage, full-body polyurethane swimsuits have since been banned from international competitions like the Olympics. That being said, swimmers seem to still prefer suits with more coverage.