Same reason Public Assembly is the most played. Its the first stage. Plus despite it being a relatively easy stage, that was one of the harder stage to get a good high score on similar to UC. Reminds me of the times when I was trying for a million on that stages as BS Jill, and my partner was a STARS Chris. Ah, the horrible memories.
It also doesn't help that UC is the only stage unlocked at the start of the game. Plus most people probably won't get the DLC unless they are willing to invest more time into the game.
Because you have it from the beginning, and without the DLC you only get three stages (That sucks). It's also the easiest stage to practice on and get used to, out of the three. At least that's how I see it.
"It's not a question of can or can't. There are some things in life you just do." -Claire Farron-