I do think the length of these courses has a fair amount to do with the selection process nowadays.
The secret is not to think about the length of them today, but whether they have any room to expand. The USGA is signing up championships 7-10 years into the future, and I don't think they have complete confidence that they can hold the line on equipment for that long ... so if they commit to a course that is only borderline long enough now, they could be screwed by 2017.
But, it's mostly about the money, and about not wanting to negotiate the terms with private clubs. Erin Hills, Chambers Bay, even Pebble Beach will give them a big discount, because they can spend the next 10 years making money off the Open exposure by selling green fees at higher prices. Shinnecock doesn't really make much money that way [apart from the caddies and their boss, of course]; so to them the hassle is the bigger factor.