The strange thing I find about MASH is Radar is the same in the show and the movie. Trapper was different, Houlihan was different, Burns was different, even Hawkeye was different. Why did Radar stay?
This article sums it up nicely:
On MASH, Radar O'Reilly truly belonged, becoming such a central part of the show's charm, it could be argued he turned out to be the franchise's only irreplaceable character.
It's a well-known fact that Burghoff was the only actor from the MASH movie cast to transfer over to the TV show. He knew both Donald Sutherland's and Alan Alda's Hawkeyes, playing off both versions equally well, and the TV series executive producer Gene Reynolds summed up perfectly why it turned out to be Burghoff's character that was an essential element of the 4077th:
“Beautiful character, very funny," Reynolds told the Archive of American Television. "Marvelous actor, very gifted actor. And he was, for us, he was like a little boy in the camp. Kids could identify: I could be Radar. They identified with MASH through Radar. So that was an enormous help.”
MASH series creator Larry Gelbart said there was a reason why Burghoff was so talented at evoking this particular character, who in many ways served as the heart of the show, with his instinctively helpful ways and teddy bear in tow.
Gelbart said, "Gary was phenomenal. He’s a phenomenal actor. He does that thing that actors are supposed to do – and not all of them do – he listens when other actors are speaking to him, or speaking to someone in the scene, and yet he’ll do something on his own, that doesn’t take away from the focus. He’s just a fine, fine actor, by instinct and by training.”
Director Charles S. Dubin, who shot more MASH episodes than anybody else, echoed this praise, and extended it to how well Burghoff worked with what the writers envisioned, too: "He was wonderful. He had all the nuances and he captured what the writers had in mind wonderfully well. He really did Radar and, of course, it was written so the moment Harry Morgan was about to say something, then he would come in and say what Harry was about to say, which is why they gave him the name Radar."