Why was Alan Alda cool in Mash, but nerdy after that?
This is one of life's biggest mysteries.
He's an actor. He acts the part and follows direction. MASH was a brilliant part for him to play. Actors don't always get such good offers.
Now he’s old and can only stomach M*A*S*H*ed potatos.
Actors play different roles.
It was the characters that he got to play. He could not be cool with all his characters he played.
He was always a left wing douchenozzle, but he kept it subtle during MASH. After that he teamed up with guys like Woody Allan and Michael Moore and went "all in" on the nerd / douche / liberal thing.
he must have auditioned for, and won, the nerdy roles - or being asked to play them.