They can't land the same number of planes when it's snowing. Something has to cancel or divert. This time, UA diverted the TLV flight. Next time, maybe they divert a flight from another city or another airline loses the slot lottery. Making assertions like "they should have landed flight XXX" have a hidden "and screwed over flight YYY instead" component, which if they had done there would be an identical post here saying "they should have landed flight YYY".

Or maybe the flight had a lot of connecting pax and UA ops decided landing in IAD (which will have fewer onward flights cancelled than NYC) would serve them better than landing at EWR with everyone in a hotel for 2+ nights.

Furthermore, LY doesn't have any ops at any other east coast station other than BOS...they are more likely to delay/cancel than divert to an airport with no ground staff if they can help it. UA diverted a flight to a major hub with ground staff and a dozen daily flights to the original destination. Seems like there are better things to complain about!