Masahiro Sakurai has been both very vocal and candid with fans who have questions about the development process of Super Smash Bros. In a recent Games TM interview, Sakurai confirmed exactly why fighting characters don’t appear in Super Smash Bros. His reasoning is that since the character is already known for fighting, it just doesn’t bring much to the table.
“The biggest feature we look at in Smash Bros. is, ‘What does this character bring to Smash Bros. that other characters don’t?’ If you look at… someone from a fighting game already, and people like fighting with this character, from my point of view, it’s like ‘this guy does what this guy already does. He fills the role that this character already has.”
It’s interesting to see his take on this and he mentions in the interview that Mega Man is a special case and is being treated a bit different because of it. He hints that because of this special consideration, other third party characters joining the roster will likely be very thin.