I think because people believe at this point it is taken that failing at Yahoo would hardly be surprising. For 2 reasons: 1) the company is in much worse shape (especially in reputation) than it was when previous CEOs failed to turned things around and 2) after a bunch of people try and fail the stigma of failing is likely to be reduced. Also here reputation is so positive that failing at a task many think is going to be hard not to fail at, won't likely turn those thinking well of her around.

Now she could fail in ways that are so bad that it does harm her reputation, certainly. I tend to agree with those that think as long as she doesn't fail for reasons that make her look really bad, her reputation won't suffer too much.

Also if Yahoo is marginally successful she may get an outsized boost - due to others failing to do even that.