Michigan simply does not do a good job of developing any particular position for the NFL, and they positively suck for certain positions such as running back, wide receiver, and (believe it or not) quarterback.
The briefest of glances at their coaching staff is all the explanation you need for this phenomenon.
If you want to look long term, the Hayes/Bruce coaching tree has been exponentially better than Schembechler’s... and it’s getting to the point of absurdity.
Who does Michigan even have to look at after Harbaugh? No one worth a damn wants to coach with him, which means it’s very unlikely anyone worth a damn is going to appear to take the reins whenever Harbaugh is gone.
And their culture (and two decades of failure) prevents anyone outside from wanting anything to do with the place as long as there are marginally comparable opportunities available.
It’s not a stepping stone position, it’s a career ender for coaches with too high of expectations and too little chance of success to be appealing (see: Rodriguez, Hoke).
Meanwhile, Tressel and Meyer both started with Bruce in the 80s, and Tressel has given us at least one promising future prospect in Fickell while the Meyer/Day branch looks to reap coaching dividends for a long time.
You could write a book about all the things going wrong with ttun, but coaching tree, both right now with no one wanting to be around Harbaugh and going back decades would be a chapter in that book.
Edit: also, hat tip to Day for walking in and taking two of their best coaches right out from under Harbaugh - that was really a knife in the kidney to start off the new chapter in the rivalry.