Because the humans have the odds stacked against them enough without worrying about those who are stronger, faster or can bend the laws of physics.

Before anyone accuses me of close mindedness, I have had my abilities suppressed and while I don't appreciate being turned human every other port, I can't ignore the implications that having abilities has on people who don't have them. How that makes them feel afraid when they're here, lost and alone and in a place that they couldn't imagine existing. That's a very isolating, frightening place to be.