I'm a runner, primarily and I ride a bike for cross-training for marathons. As mentioned, running is a lot cheaper than a bike and requires no maintenance.

Plus, racing on foot doesn't carry near the stress or danger of racing on a bike. Nobody really crashes or gets badly injured in a footrace.

I also feel like running is more of a workout. When I come back after an hour or two of hard running, I feel a lot more "worked-out" than after I come back after a similar amount of hard riding. Maybe I'm just not riding at the same intensity at which I run, but I feel like I'm riding pretty darn hard and couldn't finish the ride if I went harder.