Some time ago I had a
thread about what being a elf-friend really entailed.
Though Sam did indeed have an affinity for Elves, I don't think he is quite as singular as Bilbo and Frodo. Both of them were "chosen" to have a special, long-term role as Ring-bearer, and that choice was made with their peculiarities in mind.
Sam is a much more "normal" hobbit, his liking of elves and tales notwithstanding.
Keep in mind that it was Gildor who named Frodo an Elf-friend. He already knew Frodo somewhat anyway, through Bilbo, and discerned through speech with him more of his special character.
As for Sam, Elrond may have given him the honor later, the reader simply not being specifically informed of it.