I believe in the US, the majority of recreational skiers consider "ski season" to last from Thanksgiving (or early December) through Easter (April). I think most resorts generally schedule their season that way too (assuming they have average snowfall or better). After Easter, spring conditions tend to prevail at most US resorts. Not all skiers like spring conditions, as they can be difficult and the snow becomes effectively un-skiable for them (generally after 1pm - or sooner if a really warm day).
I think some US resorts have summer mountain bike operations that would be impacted if they tried to support skiing ops on the mountain at the same time. When it's May/June at those resorts, there's way WAY more people there wanting to mountain bike. If there's still snow up high at/near those resorts, some folks will hike/skin/climb to get some turns on skis - but those folks seem relatively few. BTW: mountain biking generally compliments skiing as it helps keep you fit & keeps you familiar with speed & body dynamics.
It's been decades since I've skied east of the Rockies. But I'll venture a guess that for many eastern resorts it's impossible to have super long seasons if it's too warm for the snow guns to make snow.
However, there are indeed US resorts that specifically support & advertise long ski seasons (e.g.: skiing in May and even June).
Whistler is kind of cool in May/June because you can (usually) do some degree of lift-served skiing on the glacier(s) in the AM and then go do lift-served mountain biking in the same day - all on the same mountain complex.
I guess if there's one thing US resorts could do to entice more late season skiers it would be for the resorts to be more aggressive & proactive in "farming" and maintaining corn snow development on their best spring snow slopes. I think some resorts (like Sun Valley & Mammoth) actually try to do this. Other resorts seem to pretend there's no such thing as corn snow.
Also, I've always wished resorts would consistently post - on a daily basis - the actual estimated time window when spring skiing conditions will likely be best for that day (eg: "Best Skiing Today: 9:45AM - 12:15PM). If they did that (and the forecasts were usually correct), more people might do more spring skiing. I'm saying this because, during the spring ski season, I always see too many people show up at the resort too late in the day. I see those people because the snow on the slopes is done for the day & I'm in the lot getting ready to head home. So, those folks arriving too late are going to try and ski on glue/slush. Many of them will not have much fun & probably won't return until the start of the next season.