Go to www.saikyo.com for much of Dan's story. I think there is more to it, but I have not played Street Fighter 3 yet, so I don't even know if he is in it.

As for your question, most people don't like him because he is harder to use than the other characters. Most of his attacks are fairly short-range, making it so that a person needs to be very good with him to win. Also, some just don't like how he acts (the thumbs up taunt), and/or his pink gi.