Harbaugh said it was "a dream of mine"

He was working for the 49ers at the time and I'm sure he wanted to keep things on the DL

Here is the quote:

“I have to tell you, it’s been a dream of mine to come back as a football coach to the University of Michigan,” said Harbaugh, who spent part of his childhood in Ann Arbor and played for Michigan from 1983 to 1986. “I have thought about that dream since I was 9, 10 years old, through my adult life. Now it is time to live that dream.”

Source:

Harbaugh announcement