Why wait?
To avoid looking precipitous and defensive, and to keep the spotlight on the substantive contents and implications of “the memo” for a longer period of time. My thoughts, anyway... 😊
Understand this is a fight for The Republic, not a janitorial task to exercise a few bad characters. Justice will be considered complete on a firing - it wouldn't be. Trump will be cast a dictator for cleaning out his opposition.
On the other hand, if he waits until the chips are really falling, the public (and eventually media - think ratings) will demand prosecution and more. Firing will be a given, not part of the discussion. Winning, truly winning, is when it becomes Trump's job to baffle the cries for guillotines and gallows.
God save our Republic.