Frank Capra was mad at his attorney, and never got around to signing the forms which would allow him to extend the copyright. So the film fell into public domain.
Once it was in P.D., it was free to everyone, and all the TV stations showed it all the time.
It is my opinion that this P.D. exposure is the reason for the film's popularity. I'm not commenting on its quality. There are a lot of good movies that aren't as popular. I say that it became popular because it was aired so often, and became a Christmas staple on US televison.
Then a court ruled that although under some circumstances a movie can go into public domain, the copyright of the music and soundtrack is covered by a different law. This ruling in effect meant that TV stations could show the video for free but not the audio.
So the movie effectively went back into copyright. It was then that NBC bought the exclusive right to show it on TV. And NBC shows it only once or twice a year.
I don't know when NBC's contract expires. I also don't know when the movie goes into P.D. again in the US - probably the same 75 years as for audio recordings.
edit for typo
I thought it was Ted Turner who boughts the rights???
m~