Sometimes those A320s have fuel load issues when headwinds are strong or there is weather en route. The flight had to fly directly north out of JFK, either due to headwinds or the weather over Michigan/Ohio, and simply didn't have the legs to make it all the way to the west coast at that point.

https://flightaware.com/live/flight/...057Z/KJFK/KBOI

EWR-SJC left around the same time, went south, and made it, but that's also a 738.

https://flightaware.com/live/flight/...145Z/KEWR/KSJC

B6 JFK-SFO left a little earlier, took a more aggressive routing, and made it with an A320.

https://flightaware.com/live/flight/...025Z/KJFK/KSFO

Everyone else flying more capable airplanes JFK-SFO went north/south around that time, and they all made it. Here's DL's 4:40 departure, which amounted to almost 7 hours of flight time.

https://flightaware.com/live/flight/...005Z/KJFK/KSFO