Because duo's work together all the time like Ken and Ryu are a duo. Chun-Li doesn't always work with Ryu and when she does it's never just him and her but a team of others.
R. Mika >> Luara >> Ibuki
Chun-Li and Sakura aren't really competitive. Chun-Li's just a cop and Sakura was a Ryu fan girl is all. R. Mika is probably the most competitive because she has a passion for wrestling next to Laura the same passionate for what she does. Ibuki come from a family of ninjas so competition was probably a thing for her as well. But, Chun-Li and Sakura as competitive -no. Sakura probably once upon a time but she kind of grew out of being a Ryu fan girl.
I don't understand why Street Fighter keep going back to Bison.
Yeah, but it has to be the continuous of Street Fighter: Assassins Fist
Chun-Li and Ryu aren't really a duo.
Cammy and Chun-Li are a duo?
To be honest -Street Fighter need a reboot series starting from the original because the game stories are all over the place and suck.