There are no nonstop flights from LGA to SLC, the furthest you can go from LGA is Denver.
Congress change the law governing DCA to allow certain West Coast flights, DCA previously had a perimeter rule just as LGA does. The main reason for Congress changing the rules with DCA was a selfish reason, they don't like driving to IAD. If you look at the main benefactors HP and AS they have strong leadership representing their hubs. Senator John McCain (R-Arizona) and Senator Young of Alaska, both high ranking memembers of Congress.
LGA has in the past had daily DC-10,A300 and L-1011 flights, and today DL still flies plenty of 767-200s, 300s, and 400s into LGA daily. So weight is not really a problem for LGA in regards to West Coast flying, as mentioned previoulsy mentioned a fully loaded 737-700, 800 and 900 weigh much less than a lightly loaded DC-10, L-1011 or 767-400.
Eastern Air lines flt # 701, EWR-MCO Boeing 757