Because it's a lead sled. Very few if any commercial airliners have the power and climb capability of the 757. Only the Dornier Jet comes to mind and it is slow in cruise.
The 321 wasn't really designed to fly 3+ hour trips. When it's full of folks/bags/fuel, it's climb capability is pretty bad.
Put on the east coast markets and it does great. Full load with half the fuel and 350 to 370 is obtainable.
Leave it to the airlines to push an aircraft to it's segment limits. The 321 gives you the same performance you would expect from a 727 on those transcon flights. Or a CRJ from PHL to MSP...same difference.