Michigan, traditionally, has taken their ball and gone home whenever the scales tipped in ND's favor. They beat up on us the first 8 times, then we beat them in 1909 for the first time and it's the old, "Well we were just treating it as a scrimmage." So Fielding Yost, the unabashed anti-Catholic and anti-Semitic he was, pulls the series because he doesn't want to get beat by ND. Blackballs us from the Big Ten and tries his hardest to make all Big Ten teams blackball us. He didn't want to compete with ND on the field nor did he want to have common opponents that could compare his program's common games with ND, which makes it harder to win a Natty. He also wanted ND to fail as a program. So we go national on the way to global. Then we play them in 1942, UM wins. So all is good lets play them in 1943 says UM. ND wins in 1943 and you guessed it UM pulls out again. Series returned in 1978 and is even since. If you take away the games from 1887 to 1909 (when ND had barely established anything above a rag tag group of a football team) the series is tied 15-15-1.
Not to mention Sparty and Purdue are much more of traditional rivalry than Michigan could ever hope to be to ND. ND has played only Navy more than Purdue. If it was my choice I would protect Navy, USC, Purdue, and one other California team. I'd play Sparty every few years if we can work in a home/away with them.
So after all this I say farewell and f*ck off Michigan.