Is it possible that they just ported over the PCE cd version of the game? That game was single player as well, if I recall correctly. Other than the Columns intro theme, it is the only other game there that has cd audio tracks.
actually the cd has alot of music tracks, I made my own version of it with turbo duos golden axe music so you get more fun out of the 1 player game. And no way is it anywhere near the duos version, the duo has TONS of Anime' and the in-game graphics on the duo is not as good. They simply took out 2 player on Golden Axe to make you go and buy the cartridge! nothing more, streets of rage is 2 player though, 2 bad its got cart music instead of cd music, but the voices and stuff are cleaned up alot!!