The question isn't "why is Ryu becoming a Power Ranger," it's "why has it taken this long for Ryu to FINALLY become a Power Ranger?" The Power Rangers: Legacy Wars mobile game previously enlisted the likes of Ryu, Chun-Li, Cammy, Guile, Akuma, and M. Bison, and now the Street Fighter x Power Rangers action is making the leap to the real world in a new promotional short that was announced at Comic-Con.
Power Rangers: Legacy Wars Street Fighter Showdown—which will have Jason David Frank returning as Tommy Oliver/Green Ranger and Ciara Hanna as Gia Moran/Yellow Ranger—is due out this fall. As you'll see in the trailer below, the live-action film has Ryu taking on the responsibility of becoming a Power Ranger himself.
To get everyone ready, Ryu Ranger is now in Legacy Wars, and you can see him in action below: