They hate him cuz his nose is big and a different color, his girl's dad goes rabid when he finds out Rudolph and them are together; a clever way to address racism in narrative for 1964 mainstream TV culture. Classic.
Along with "Grinch", probably the most iconic animated Christmas special still holds up pretty well even after over half century. Everlasting story of misfits is told in a way that seems charmingly old-fashioned now, but it was probably subversive in its day - thus making it welcomed for every future generation. Creature and scenery design appear like a blueprint for "Empire Strikes Back" - which adds an extra flavor