Because it was not a sending off offence.
It wasn't an elbow, it was side of arm placed to the side of an opponents face, similar to what you see 50 times a match when a central defender and centre forward challenge for an aerial ball.
It wasn't "thrown", wasn't an elbow and wasn't with excessive force. It just does not qualify as a red card offence, which is also why VAR quickly cleared it.
Was just about worthy of a yellow as it can be classified as reckless rather than just careless, which would mean no card at all, but really it is borderline yellow/no card, not close to being worthy of a red.
I, along with all Ref Watch panellists and about 80% of the Sky Sports community who voted all thought that it wasn't a red card offence.
Whereas they all thought, as did I, and the public voting did with a large margin also that the Cavani disallowed goal should have been allowed=, and VAR should not have got involved.