I think it would be quite easy to have less revealing attire for beach volleyball, using thin lycra for slightly more coverage with a similar comfort level.
But the bigger point here is that there are existing and established culture and 'norms' in beach volleyball.
So as a very rich country wanting to use that wealth to bring basically 'foreign' sports to your country, you don't tell those sports what to do, you take it as it is or leave it.
I'm reminded of the Football World Cup which was held a few years ago in the US: quite early on after the US had been awarded the gig, the US TV networks were demanding that instead of 90min in two halves of 45mins, that FIFA should allow 'quarters' like in American Football, so they could get their usual and expected amount of advertising time

.
Quite rightly, they were told to fuck off.