Michael was going through a lot of personal stuff in the 90s, so I doubt he wanted the added stress of touring with Queen on a full time basis and replacing a legend like Freddie.
The surprising thing to me at the time was how he kind of fell of the map here in the States for much of the 90s. After the success of Faith, I thought he'd be THE dominant 90s pop star, or at least for the early 90s, but it didn't turn out that way.