Cuz he lied to Pence, who then vouched for him.
That left Pence looking like a dunce if they kept Flynn.
This way, it looks like Trump/Pence don't put up with unreliable troops.
If Flynn didn’t go, it would look like the Admin was covering for a someone who can’t be counted on to remember critical information.
A man at Flynn’s level, simply can’t forget to include information, about anything. His position is high enough that he doesn’t have that luxury.
Trust is imperative, and he would be a distraction now.
Resign. Move on. Appoint new.