LGA wasn't that bad as far as weather at that time... it was a large line of thunderstorms and an Air Traffic Control "airspace flow program" through that front. LGA wasn't the only airport affected.

F9510 http://flightaware.com/live/flight/F...645Z/KDEN/KCAK Yellow and Red are thunderstorms.

UAL310 http://flightaware.com/live/flight/U...650Z/KDEN/KLGA

1) Just a wild guess that the difference in metal A320 vs B757 or the way they were dispatched gave United enough range to fly totally around the front. It looks like they both started with a similar route but Frontier for some reason midway tried a more direct route. Maybe actual fuel burn was more than planned and of course the possibility of a diversion to Canada isn't really an option. Pilots and their dispatchers coordinating with ATC would make a route change and diversion decision in such a case.

2) LGA could certainly have been packed with aircraft waiting for departure clearance with the AFP in place. So even if you could get in would you get a gate? (how many does F9 have there) Here's an instance where the "passenger protection" from the 3hr tarmac rule can effectively force diversions or pre-emptive cancellations. Millions of dollars in fines for each flight at risk. Airline groups did say that would be the unintended result when the rule was being debated.

Frontier wasn't alone... AA370/AS1061, BA6625, AX/US3514, MQ567 diverted with many others late arriving or cancelled within the scheduled TOA 4pm-5pm. Things got worse after that.

Weather and ATC are considered uncontrollable.