1st - BC is a private school, and fits well with Miami, Duke, Wake Forrest, and Clemson. They don't match up well with the Big10 "land grant" universities for many of the same reasons Notre Dame doesn't match up well.
2nd - BC is a hockey school. And moving to the Big 10 would force them to join the Big10 hockey league. Thus giving up their Hockey East rivalries with UNH, Maine, Notre Dame, Northeastern, UMass, etc.
Honestly, if Big10 wants to be in New England they should be going after UConn.