Sasuke has been a popular character since the beginning, due to him being a main character, and an interesting one.
When newer characters who come in the same category of: Bad-ass, mysterious/cool, and skilled. People who love these types of characters have more to choose from than just Sasuke. They can get interested in characters like Neji, Madara, Kakashi, and so forth.
Like someone pointed out, when these characters become less interesting, or die out. The fan's can move into a different demographic. Personally I think it is a bit of a stretch to name Sasuke the best character of the series, but he is good. Kishimoto's writing from start to finish on Sasuke has not always been 'perfect'. I think the character who received the best development from start to finish and still remains interesting would be Jiraiya.
Too bad most of his fan base left.