Actually, it's really mostly fashion. A quick-release hub is quite capable of holding a fixed-gear or singlespeed wheel in place. However, track bikes use these big ol' nuts to hold the wheel on, so that's what's popular. Add to that the fact that too many people don't know how to adequately tighten a QR, and the belief that a QR somehow won't hold a fixed wheel tightly enough was spawned.

So, in sum, the answer is: fashion and a misguided belief that a QR isn't good enough.