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.
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
Trump isn't exactly big on forethought.