I don't think the SF characters are better, I think they are easier to play with. We all know how Ryu plays, but we don't know how Law plays in a total new environment for him. Once we take the time to learn, they will be as good as SF characters.
Common, it's easy to see, Heihachi has overheads in his strings, LOTS of Tekken chars have that too, that's a godlike thing that has to be explored, but most people seems kinda lazy about it (or just don't like the game) and just pick Ryu/Rolento/Ken.
And, now with the nerfs on these 3 characters, maybe people start trying the Tekken ones.
And I gotta say, I love the Tekken characters, they are one of the reasons I still like this game a lot.
They are fun as hell, I love the strings, the combo possibilities, they are great!