I made a hype video.
How did Michigan get a top five NFL coach to come back to college? There's actually one word for it: passion.
The school is so passionate about football that it was willing to give Harbaugh whatever tools and control he asked for. The big money donors are so passionate that they offered to finance NFL-/Saban-like money for Harbaugh and a top-echelon staff. The fans are so passionate that they flooded onto every message board and commented on every medium every time some writer or TV person seemed likely to breath so much as "Har…" Jim's friends and former teammates are so passionate that they reportedly formed a train of callers and kept up an unrelenting press since November. And finally Jim is so passionate about his school that he gave up/set aside his (apparently real) NFL aspirations to come back to the farm.
College doesn't have the NFL's money, nor its cachet, nor is it the pinnacle of the sport. But the thing about college football is we care way more. That's why it happened.