To attract more male audience, in case they think of it as a girls game, then they have a reason to watch >.>
Also, as the others said, to help them move faster and easier and it keeps them cool...and I guess stops them having sticky shorts? =S lol ^-^
firstly its because it allows them to move freely without anything flowy like a shirt getting in their way or tight that might condtrict movement. it also seems to be for attracting more men to the games
Because the designers are male, and they like to see the legs on all those cute beach volleyball chicks.
For the guys watching of course;) haha but seriously it's probly to let the players move faster