More than any of the other conferences, the B1G is controlled by its collection of University Presidents, even more so now that Jim Delaney's retired.
It was the Presidents who made the initial decision, and it was the Presidents who misjudged the push-back they'd receive afterwards. They assumed everybody was eventually going to follow suit. Once it became clear that the other Big Dawg powers were going to play come hell or high water AND (most importantly) bring in the big TV bucks for their schools, the Presidents found themselves between a rock and a hard place with no real choice but to relent.
Anyone who thinks this was all on a league commissioner in his first six months on the job doesn't understand the wonderful world of academic leadership in colleges and universities.