https://screenrant.com/trapper-mash-exit-sad-goodbye-reason/

Wayne Rogers' Trapper John was among the original cast of MASH who left early on despite its success, but his departure is the saddest of them all.

It was sad but I've always preferred BJ. Alan Alda said that Trapper wasn't working for the show because he was too much like Hawkeye.

I think all the character changes that gradually took place were for the best.

While I like Mike Farrell, to me, Trapper was the original role.
Great guy but, of course, not really intended to be anything more than Alda's sidekick.
Same with Beej.

Both are good.

Here's how I like to state it:
MASH was best when Larry Linville was on it. That way, we get the best of both Trapper and Beej.

Not sayin' "Chas" wasn't any good. DOS was great, but Frank was one heck of a character.