From a Smithsonian Magazine online article:
When the special finally aired in 1964, it became such a hit that it has been rebroadcast every year since, making it the longest-running Christmas special in history. Even today, the special still punches above its weight; when Rudolph the Red-Nosed Reindeer aired on CBS in 2016, it beat every show except This Is Us. In 2017, more viewers tuned in to watch Rudolph than A Charlie Brown Christmas, which ran on ABC in the same time slot.
Reindeer and dentists, puppets and LED light bulbs, Gene Autry and General Electric—these odd pairings might not seem to have much in common. But each played an important role in the making of Rudolph the Red-Nosed Reindeer, a classic Christmas special currently celebrating its 55th straight year of annual reruns. Before Rudolph lit up the small screen, a series of tragedies, twists of fortune and lucky coincidences allowed his tale to endure through decades—eventually ensuring a place in holiday tradition.