Like @Yacht said, back then before global when it were region days, everytime Colorado area was featured in Expert Server (The most crowded airports in Colorado were KDEN, KASE, and KEGE), both KASE and KEGE often received unrealistic amount of traffic as people spawned in 777s and even A380s. So the developers tried to put TFR (Which is enforced especially when ATC is active) to limit those who spawn in heavies.
Nowadays after Global, both airport are rarely used. People only use both airports for regional flights as there are many airports to choose now.
Both airports used to be one of the busiest airports in Expert Server if they had ATC controllers there.