Why wouldn't the Big Ten take them now? Seems inevitable.
ND is the most valuable property not already in the Power 2. Guess the Big Ten wants to make sure they get them. And to keep an even number balance, they'd only be able to take one of Oregon/Washington along with ND. And if Jeff Ermann is to be believed, it sounds like it'd be Oregon (gotta think that's the Phil Knight influence at work).
Thing is, I don't think ND is really inclined to make a move just yet. They'll wait and see how CFP expansion shakes out. If it looks like their chances of getting in the Playoff are still decent if they stay independent, they will. If they think they're going to end up locked out, they will join a conference.