There is also a component to this that has to do with cascading delays. Many regional jets fly out-and-back patterns within one region of the country, while mainline aircraft move around more in the system. So there's little point in prioritizing that regional jet from PHL heading out to a small city, when it's just going to turn around right away and be delayed again going back to PHL (for the same weather), while a PHL-LAS flight may be continuing on to PHX unaffected by east coast thunderstorms, or a PHL-LHR will be turning back to PHL but a whole day later (thus avoiding the current weather).

In fact due to this out-and-back pattern when the delay gets long enough they just cancel the two-segment PHL-XXX-PHL part of the trip and that aircraft can be on time for the rest of the day if the weather clears up.