As a very astute person recently pointed out:
Because firing a Special Prosecutor worked out so well for Nixon.
Even Trump’s own party has threatened impeachment if Sessions is fired. Trump floated the idea and got enough blowback to heat Antarctica.
Mueller has promised more obstruction investigations if Sessions or Rosenstein are fired. The anti-Trump trio has laid the trap, and it’s an insidious one. Indeed, it’s a coup attempt. I pray Trump survives it.
Even if you think Rosenstein and Mueller are part of some nefarious plot to overthrow the duly-elected POTUS (and I suspect Mueller probably is) it’s absolutely goofy to accuse Sessions of being part of a coup against the president he supported and whose agenda he is implementing.