I have the answer to that question.
Motorcycle riders wear black because motorcycles are cantankerous beasts, always spurting oil, grease, and grime all over their rider, and black doesn't show oil, greast, or grime.
What, you thought it was because of a desire to look cool or somethin'? Silly you!
Oh, BTW, I found this out the hard way, when nice safety-yellow riding gear ended up being more grime-blotch-grey... and that sh*t don't wash out, either, it soaks into the threads and permenantly stains them. Bummer!
-- Badtux the Motorcycle-Ridin' Penguin