just an unbelievably well written show..
Larry Gelbart was the head writer for seasons 1-5, after season 5 the show was less funny, just as good, but less funny.
Gelbart wrote the one liners for Linville (Frank, who left after season 5 with Glebart and Winchester came in to replace Linville) Hawkeye and Trapper John.
I used to write a little comedy for a radio show back in the day, it's one thing to write gags, but it's another to deliever them.
Alda and Rogers could deliver one liners like Henny Youngman and Groucho Marx, that's how good they were.
It's all about timing in comedy, any comic worth their weight in salt will tell you that, so it's no coincidence that without the Gelbart and his writing team, the setup man (Frank Burns) and his foil Trapper John, the show would be less funny.
That's when Alda got more involved behind the scenes, the plots were just as good, if not better, but just less comical.