If you talk to somebody like Rich McGarry who operates the place, you'll hear words like "We'll open when we can offer a quality product". The last couple of years, quality product was defined as top to bottom skiing rather than a pickup truck ride to ski Rime. This pushed the opening date out since it's tough to blow snow on lower Bunny Buster in October and early November.
I presume this saved Killington money since labor and equipment wear and tear running pickup trucks, the Canyon Quad, and the Glades triple is tough to recoup on steeply discounted day ticket sales. It's not like you can charge $70 for 1 trail and 600 vertical feet.
I guess you could argue that with all the dirt cheap season passes, 1 top to bottom trail doesn't cut it either and they need to have enough acres online to spread out all those people. If you were at Killington on a Saturday in November this year, the trail congestion was the worst I've ever seen. If I were Killington's liability insurance provider, I'd probably refuse to cover them since they've gone and created a very unsafe product.