I FOUND SOME INFO....I don't know the exact reason, but this link clearly shows the delays at LAS.
Looks like 50% of flights leaving LAS are "Very Late" and about 55% of departing flights are "Very Late." Only about 20-25% are on-time.
Link to LAS
http://flightview.com/TravelTools/Vi...sp?airport=LAS
Link to ORD: (virtually everything is late there)
http://flightview.com/TravelTools/Vi...sp?airport=ORD