The two reasons it is not is because Tim Conway, as Ensigh Charles Parker and Joe Flynn as Captain Wallace B. Binghamton are still engaging in their zanny antics. The rather strange title comes from the idea that Parker is mistaken for a hot shot fighter pilot in the Air Force (which, of course, was still the Army Air Corps during World War II, but for some reason historical fidelity was not high on the producer's list). Of course, every time Parker screws up he gets promoted. Obviously, with Borgnine apparently on vacation (the series would air through 1966), Parker and Flynn get more slapstick to do, but the script by John Fenton Murray, based on William J. Lederer's story, actually slips in some satire to go along with the physical comedy.
Is 1965's "McHale's Navy Joins the Air Force" better than the 1964 "McHale's Navy" film? Well, yes and no. This one is actually funnier because of the free reign Conway and Flynn have, which is a good thing, but getting so far away from the original series (and its star) still bugs me. But despite that potentially fatal flaw this odd little film is actually an enjoyable experience for fans of the series. For others it is a reminder of what military situation comedies were like between "Sgt. Bilko" and "Hogan's Heroes" (i.e., before "M*A*S*H").
Actually, that last aside should be more specific in reference to the early couple of seasons for "M*A*S*H" when the series was very much in the mold of Bilko/McHale/Hogan. It was not until the classic episode "Sometimes You Hear the Bullet" that we first saw how "M*A*S*H" could become television's first true dramedy.