[mistercow]

Well, a few reasons. Broomstick riding is supposed to be hard, so unicycle riding seems like a reasonable analogue. Another reason is that it has the similar aspect of falling off, while still being extremely maneuverable.

And lastly, I have this mental image of a nerdy guy peddling a unicycle like hell away from a 20 lb flying medicine ball, while another player cruises over to smack it out of the way with a baseball bat. There is no way any alternative is going to live up to that vision.