Originally Posted by iSpyCams

Serious answer:

I think it's because Americans have odd ideas about their bodies and swimwear that exhibits the outline of your junk would be considered vulgar by most and "show-offy" by the rest.

To complicate things I seem to remember in the 90's in Austin Texas speedo's and similar swimwear were embraced whole-heartedly and seemingly overnight by the gay community in a way that made it a strong statement of sexual orientation to wear anything but trunks or bermudas while swimming. So even in places where it's no big deal to be openly gay, it's not advantageous in any way if you aren't, so trunks it is.