Why are they almost-always wearing a bikini?
The official explanation:
Beach volleyball is a sport that was developed in Hawaii, southern California, and on the beaches of Rio. You play, it's hot, and then you go and jump in the water.
One-piece bathing suits won't work because the sand gets inside and chafes. So the less material that can trap sand the better.
It's also important that the straps don't restrict movement or breathing.
Beach volleyball matches also proceed in all kinds of weather including rain,
and even when there's a downpour.
Just another reason why they wearing a bathing suit.