It's because they're so obsessed with the "Michigan Man" myth that they didn't think it was possible for Hoke to fail, particularly after he (unlike Harbaugh) got off to a fast start on the recruiting trail. So they continued to look the other way on Hoke's lousy career record, his (to put it lightly) questionable decision-making with several off-the-field issues with his players, etc.
This obsession is only magnified with Harbaugh, who actually has some track record
of success and who suited up for Michigan. The expectations are out of this world, and that's saying something for that fan base. I honestly think the majority of them would prefer Harbaugh, who hasn't actually won anything and who is off to a thud on recruiting, to Meyer, who has multiple titles (not to mention a current roster and yet another recruiting class that has OSU, to put it lightly, well-positioned).