My understanding is that ND's desire to remain independent was three-fold - one it made them special to be different from other power programs in that respect and two they did not want to be confined regionally, as they recruit nationally and their fans are dispersed nationally. Third, they had important rivalry games that they wanted to keep - the most important being USC, Navy, and Stanford. The ACC was something they were willing to affiliate as it allowed them to cycle through a limited number of ACC games (of which a few teams are also traditional rivals in BC, Pitt, Miami, and Georgia Tech) and still play their traditional rivals plus a few other national games. The B1G arguably had more important rivalries in MSU, Michigan and Purdue (and arguably NU and PSU as well) but we would have required them to join as full members.
With USC, UCLA, and possibly Stanford joining the B1G, we would solve 2/3 of those issues - the national reach along with keeping most of their rivalries intact (including probably making the line up even more traditional in that sense by switching from the ACC rivalries to the B1G rivalries). Its also for these reasons the chances that ND ends up with the SEC instead is basically between zero and nil.
My guess is that ND will end up joining the B1G, but they will hold out as long as they can, given the first reason, which is also flavored with a bit of emotional responsiveness to the years of Fielding Yost doing what he could to keep ND out of the B1G when ND actually had wanted in.
