Turns out, that was not always the case.
The film was a box office flop when it premiered and no one bothered to maintain the copyright in the 70s. So, cable channels could play the movie as many times as they wanted without paying for it.
Through repeated airings, it became a Holiday classic.
All of that changed in the 90s, however, when NBC received exclusive rights to the movie.
Which is why it only plays on NBC a couple times a year.