Well its simple isn't it. The people that move up (yes up) to a fixed gear, are usually fit young guys who have had heaps of years experience with other bikes, and probably commute as well. In other words, really strong riders.

The people who ride road bikes are usually weekend cyclists who don't commute and not as young or as fit or a man. (cause admit it how many girls ride fixies). In other words, not as strong riders.

The other reason is that gearing and freewheel makes you lazy. Its like the old saying: Why stand when you can sit! Why struggle in a high gear, when you can coast or ride in an easy gear. Riding in a 48-16 or something makes you a strong rider for the simple fact that to maintain a decent crank rotation speed, you need to be going to 30 - 35 kilometers an hour, and a normal cyclist just doesn't keep it up that high.