Because Trump is incredibly shrewd. I know people like to think they're smarter than him, but he's an incredibly sharp individual. If I were in his shoes, I would have declassified and dumped all of this months ago--however, he's playing it much better. He's having a "hands-off" approach to this. That way his critics cannot have anything to point to him ceasing the investigation into him. He knows he cannot directly fire Mueller without that starting impeachment proceedings. So, he's having Congress do the work for him. Methodically, the truth is coming out and is grinding away at the Democrats' narrative. Memos are coming out gradually that are neutering the Mueller investigation.
Many people ridicule Jeff Sessions for his impotence. I differ. I think he's behind the scenes disarming the Deep State. He made the mistake by hastily recusing himself, but perhaps in the long-run it was for the best.
I've always thought Trump was either incredibly smart or incredibly lucky because many things seem to break his way. Remember, this guy not only beat Hillary Clinton with Obama spying on his campaign, but he bested 16 or 17 other of the top Republicans in the primaries with the RNC rigging it. I think he's very shrewd---I think it's waaaay too coincidental that Rosenstein appointed Mueller, and based on the talk around this FISA memo, Rosenstein will be fired, and based on the IG report about Uranium One, Mueller will be implicated. This story is way too perfect to be coincidental---I have to think Trump is having a hand in all of this behind the scenes with Jeff Sessions.