I don't know much about the Street Fighter stories, so this is purely from a gameplay/physical character design/basic role in the game/why Capcom created them viewpoint.
Sakura design meeting: "Hey, instead of trying to put some REAL effort into character design, let's just put boobs on Ryu!"
Ken was an alternative, not a boring knockoff.
I don't get Akuma. What was so wrong with M. Bison that they had to create an evil Ryu to take his place as main villian of the Street Fighter universe?

Where are these lemmings going? The
Super Nintendo Super Shire! Hop in line and follow them there!