Probably because, while Darth Maul hasn't much of a personality, he's more frightening and ominous, and more of what people expect of a Star Wars villain. I enjoyed Kylo Ren as a character, but some people like a character that isn't so childish.
Because that opinion may not be formed solely off of the movies, Darth Maul didn't die in and came back in the clone wars so we got to see a lot more out of the character.
