The rats have threatened him with Obstruction of Justice if he does.
The Senate Republicans have threatened to not fill the seat if he fires Sessions.
If Trump does either, he becomes the story again, not them.
I don’t believe for a second that it has a thing to do with immigration. The Dem’s aren’t going to pass an immigration bill regardless of what Trump does.
Me either, but the big items on his agenda were border security, taxes, and Obamacare. The budget has become a necessity for a lot of reasons. Without the 60 in the Senate, he won't get cooperation. McConnell won't go nuclear.
The article I linked discusses how Trump can fire Muller, then Congress could conceivably pass legislation to reappoint him. That would be comical.