Sarcasm whoosh? Your arbitrarily claiming that these characters are easy. Ryu is as much of an introductory character as Bison, Akuma, or Dhalsim is. It’s not hard to learn any characters moves, but not a single character is easy to play at a high level.
And Ken an introductory character? Like hell. You seem to be confusing “Having relatively few easy to perform moves” with “Introductory.” What exactly is introductory about Ken? He relies heavily on kara-throws, effective pokes, and a rushdown that isn’t particularly easy to perform on people… Akuma has a hell of an easier time doing what he does than Ken does, I mean hell if you want a -challenge- I’d say play Ken in that regard.
Point being, an introductory character is whoever you happen to pick up. Every single character is designed to be easy to get into and get the jist of. Every single character has enough depth to constantly learn new things about them and have an extraordinarily high skill cap.
THAT is good business sense and game design, and they followed through on that design well. Not a single character is difficult to learn the basics of, every single character is difficult to master.