“It’s a Wonderful Life” wasn’t critically acclaimed or popular when it first came out. It became well known only after its copyright expired and TV stations began airing it. There was no fee, so a lot of stations aired it year after year, and people slowly began liking it.
So the lack of copyright is what made it popular. More protectionism would have kept it underground forever. Unfortunately, due to a copyright quirk and USA’s retroactive extensions, a studio owns “It’s a Wonderful Life” again, even if it owes everything to the Public Domain.