Arooo? You do realize JFK's runways are very long compared to LGA's 7,000 footers? Of course those planes can get into/out of LGA all day long. But an A320 approaching max takeoff weight on a hot day in summer or wet/contaminated runway (summers and winters) and you're not going to take a full load out of LGA to SFO/LAX. We'd have some pretty decent weight restrictions. Heck, even without the wet/hot/contaminated conditions, depending on winds, you could still be weight restricted on a 7,000 foot runway going to do a transcon.
Case in point, we do BOS-SFO/LAX, and yes that is a longer distance than LGA-SFO/LAX, but still to make the point, there were a couple times we departed runway 9 (7,000) and went across the country. But there were many times we were just too heavy to make that runway work and had to request a longer runway at BOS.
When we first started SFO-DCA, we used A319s. And not just any A319s. They modified the engine computer chip for more power. Currently, our A320s are 27,000 lbs thrust each engine and A319 is 23,500lbs each. The modification they made was to make several of our A319s to a 27,000lbs thrust engine. So we basically got A320 power in an A319. Now that was a rocket! We used these to do full flights into/out of DCA going to/from SFO.
But then they realized the 119 seat jet wasn't enough to meet demand and we needed our 146-149 seat A320s on that route. So, we switched the route to A320s and soon thereafter all the A319s were de-rated back to 23,500 lbs thrust. It was fun while it lasted!