I saw interviews with past hosts that it's really hard to get someone to host the Oscars. No one wants to do it.
Bill Crystal, when asked why he wouldn't do any more shows said it's a LOT of work, even for the one night. The hosts have to come up with the opening number or jokes, the theme for the night, weeks or months before the show. The hosts and their writing team never make enough money for how much they put into the show before hand, and live during the show if something comes up. It's very stressful. Then when the show is a snoozer or dud, they get the blame.
I was actually surprised that Jimmy Kimmel did it again last year.