He was too much of a wildcard. Trump wants to wrap up the Russia thing and he needs someone who is more subservient to do that.
This move really doesn't make a lot of sense, I don't know what Trump gains here other than a few weeks of pretending he won.
Trump isn't exactly big on forethought.
I believe the senate only needs a simple majority to approve the nominee, though, so it might not be that difficult for them to get rid of the acting head if they feel pressure