I think the key part of the statement was "a longer window of sustained cold temperatures to start making snow".... snowmaking pond temperatures definitely matter, intake temps. Ground temps, eh?
I think it's just the forecasted warmth and if I'm Killington, I know that when I turn on the snow guns, people start looking for an opening day announcement any time and they knew it wasn't going to happen. Whoever has their ear for weather forecasting also plays a big role (I have some experience here, ha). If that person is pessimistic with warmth coming, and head of Ops trusts that individual, that can sway it pretty easily. They are waiting for their trusted confidant to say, "yes, its go time and it can be done based on what I see."
My guess is they just didn't want to ignite the "opening day???" frenzy given the upcoming pattern. I also think as IKON and EPIC passes have made skiing more mainstream, the idea of opening for a day or two and then closing down is not a path they want to go down. As hardcore skiers we all understand it... but there are a LOT of less informed skiers and riders that see the opening announcement and then don't bother checking again (sort of like a theme park, park opens and then stays open daily). Those folks start arriving at the mountain looking to ski only to find out the mountain has been closed for three days and likely won't reopen for another week or two. I think those days of opening just to open and then close are likely coming to a close. You open when you know you can stay open.