It's efficient space-wise as well - a city-centre gym can fit squash courts more easily than tennis/volleyball/basketball.

I didn't believe this about basketball, since it accommodates 5x as many players, but it's true: a squash court is 62 sqm and a basketball court is 420 sqm. OK, you can't tile squash courts with no wasted space between, but you normally need some space out of bounds of the basketball court too.

It's a class-segregated sport in the US (not sure about elsewhere). This is called out in the tongue-in-cheek-but-mostly-accurate Official Preppy Handbook. For some reason it is "higher class" than racquetball, and, sure enough, racquetball courts are much easier to find in non-rich areas. Perhaps this is a case where the general principal from the same book holds true: the smaller the ball, the more Prep the sport (squash balls are smaller than those for racquetball).