Jet engines lose performance at high temperatures and elevations. LAS is 2170' high, and temps in the Nevada desert often top 40 C.
The other factors, other than runway length, could be the pavement strength of either the runways, taxiways or aprons, or the available size of parking aprons. LAS mainly deals in internal flights with smaller a/c such as 737, and bizjets.
Simple answer, longer runway = Heavier max landing/take off weight, but life is never that simple.