Keep in mind that MASH was meant to be an anti-war/anti-military political statement, so clearly all the "cool guys" wouldn't be "Army Material" and thus wouldn't have raised up the ranks whereas Frank Burns (and to a lesser extent Margaret Houlihan) was an archetypical right wing, jingoistic American douche so of course the evil Army would promote him.
The only reason Blake and Potter were Colonels (and still sympathetic characters) is because they needed to have someone in charge.