I've tried to understand the genre as a whole, and it's definitely not one of those things that's for me. Then again, most competitive multiplayer is not my cup of tea, which is most likely why I'm not a huge fan of the genre (but still a casual fan mind you.)
I've been a big time proponent of the fact that fighting games need to evolve past their multiplayer roots (hey if single player games need to have tacked on Multiplayer or decent multiplayer, Multiplayer games should do the reverse.) I'm in it for the single player experience, and that's not because I'm terrible at them and think they're stupid, etc. It's because I don't feel the desire to practice at the genre/games in order to play on a competitive level. With Super Smash Brothers, that happened as a natural evolution due to it's simplicity, there wasn't any "down, down-forward, forward, high punch" stuff involved, and thus why it quickly became my favorite game in the genre and/or hybrid-genre. But back to the point, I'm always looking for a satisfying single player experience in a fighting game and few have provided it.
My comments about the genre's early years are something I assume to be true, as I recall reading somewhere (great way to site my sources I know) that Street Fighter or the earlier Street Fighter II weren't built to be balanced or even have a "combo" system and it was something that the players sort of created on their own that Capcom eventually implemented into the series.
the core of the depth of most fighting games come not from the game modes, but from the fighting engine itself. there are some games with a decent single player mode in them, but ultimately most companies know that the audience that they're going for, and thus the audience most likely to buy their games, is in it for the multiplayer. the genre as a whole does not need to "evolve", at least in the way you think it should. it'd be nice if a decent single player mode became standard for all fighting games, but honestly if time and resources were better spent on refining characters, combos, and such to make the game more balanced then that's the direction they should go.
also I don't know about the Street Fighter II trivia, but it's fairly obvious that Capcom either wanted a balanced game in the first place or quickly caught on to what the players want. each new edition of Street Fighter II weren't just money grabs or just added new characters, they were filled with subtle improvements over the last game. each character became more and more refined with each new version in an attempt to make the game more balanced. they basically giant patches.
in the end fighting games have a lot more depth then you perceive them to be. they just don't seem to have what you're looking for.
as a side note: most developers that tack on or add multiplayer to their single player games do it because they think the audience they're going for wants that or they want to appeal to a wider audience. that doesn't mean every single player does that though, or should be required to.