Y'know, I can see Batman joining a circus - and Aquaman - and maybe Wonder Woman. But Superman, Green Lantern and the Flash? That just ain't FAIR, man. How are all the other performers going to compete with that? Circus acts take huge amounts of skill, time and practice to perfect - you spend weeks rehearsing, for example, your new high-wire trick, getting it just right, and then along comes Superman, and you might as well not have bothered - nobody's going to pay attention to you when there's a flying man who can juggle elephants in the ring. Batman and Wonder Woman are different - Batman genuinely IS an incredibly skilled, circus-level athlete, as is WW (a few extra powers aside, which she needn't exhibit), and while Aquaman has powers, they're not flashy enough to unfairly upstage anyone. But Superman? The Flash, who can race around the tent a thousand times in a half-second and play a game of baseball with himself? Green Lantern, the human lightshow, who can do magic tricks that would make the greatest magicians in the world hide their faces in shame? Just one of those guys could upstage every other performer singlehandedly - with three of them, there's no way the others can compete. The clowns will be booed out of the ring, the jugglers will be ignored, and the trapeze artists will be greeted with cries of 'bring on the Flash!' Part of the draw of the circus is that these are regular people who have trained themselves to do these incredible things - adding superhumans to the mix is just unfair on the regular people - they might as well quit right now, 'cause no one is going to pay any attention to them.