If you can't see why an administration firing the person heading up the organization that is investigating the administration in order to appoint their own person to head up that organization is preposterous then I don't know what to tell you.
It's politically tone deaf at best and at worst it's a subversion of our political system the likes of which this country has rarely ever seen.
And since you seemingly buy whatever is shoveled down your throat as long as it comes from someone with an R next to their name, why don't you watch this and explain the rationale for firing him based on what happened with Clinton:
https://twitter.com/existentialfish/status/862109387921526784