newbie question about offroad race types

I basically understand what an enduro is and how long, and I basically understand how a hare scramble works and how long it is. But I don't understand what dictates bike selection? Is it the race type or the terrain? For instance, I see Mike Lafferty ride his 450exc in an enduro race and ride his 450mxc in a GNCC race. Most others are riding motocross models in the GNCC. Do most people ride motocross type bikes in shorter races like hare scrambles? What types of races would one ride an enduro type bike like a KTM 250exc? I realize that most of us ride what we have, but I am asking from a theoretical point of view. I have a 250exc and am curious what type of race it would be best suited for.

Also, how long, and what are the distinguishing characteristics of GNCC races and the WORCS races?