I don't know. It may just have been that they didn't like that style of humor. They're still trying to make "parody" movies now, but they're doing it absolutely TERRIBLY (see Meet the spartans/Disaster movie/Scary Movie 2 on). Do those girls think Mel Brooks movies are funny? Do they like the Naked Gun? I never felt Airplane relied very heavily on referential humor (knowing the source material). It might have added to it, but it didn't detract from the film not knowing it. I had never seen Airplane 1973 (or whatever the title is) before I saw Airplane, and I first saw it many years after it's release. I was a fan of silly humor though, and have always enjoyed those films. To me, the joke wasn't the reference to the original film, it was the entire plane lining up to slap that hysterical woman (including nuns with baseball bats). It was every passenger hanging themselves halfway through his sob story.
There's still a huge market for Monty Python, so there's still a market for Airplane. It might just not be teenage girls.