They didn't give it enough time between Red Sox games to fix the field and the field wasn't covered up properly enough because of the huge stage they had and the entire center field had to get redone because of it being torn up. This within 24 hours, mind you.
I suppose that it would be possible to prepare better by having the entire field covered, like they did with Safeco...but that's a lot newer than Fenway. The asking price I'd imagine is insanely high so they'd have to do a legit sell out to make it possible.
Also, weather is a factor, obviously. And I doubt the people in charge of Fenway wouldn't be exactly hot for the idea, especially after what happened this past summer with the concert.