Why was Alan Alda cool in Mash, but nerdy after that?

Mr Obvious

This is one of life's biggest mysteries.

Barry

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.

Anon

Now he’s old and can only stomach M*A*S*H*ed potatos.

Squidmaster

Actors play different roles.

Kathy

It was the characters that he got to play. He could not be cool with all his characters he played.

Homer Bufflekill

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.

Daytonᴱᴺ

he must have auditioned for, and won, the nerdy roles - or being asked to play them.