Originally Posted by grolby
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.
It maybe possible to get an old style steel QR to tighten down enough to hold a fixed wheel in place but I know I could defeat the new fangled Al and plastic QR. Even with a good QR I wouldn't trust it without a chain tensioner in place. A tracknut or a Phil style bolt can provide a very secure attachment using the leverage provided by a good wrench, while the QR is limited by the leverage provided by the much shorter QR lever acting over only 90 degrees. Tracknuts take about 10 additional seconds to use during a wheel change, I think the security provided is well worth the extra time.
Craig