There's actually many reasons. But this, from the Chicago Sun-Times, is one really good reason:
Notre Dame fans want the best coach possible, yet they keep scratching all the top candidates off their list. Mike Leach is a Mormon. Jim Harbaugh is divorced. Jon Gruden had a DUI. Randy Edsall hasn't won enough, Pat Fitzgerald isn't old enough and Skip Holtz doesn't recruit well enough. Then there's the latest controversy that's lighting up the message boards of what might be the most plugged-in fan base in the nation: Cincinnati coach Brian Kelly is said to be pro-choice. Where precisely Kelly stands on what should be a private issue is uncertain, and frankly nobody's business, but if true it could be a deal-breaker for many on the Catholic university's campus.
God, I hope they fail forever.