The Alpha series was way more popular than the 3 series back then. They were just bringing the Alpha people who didn't appear in SF4. Except Sodom, who's yet to appear on a 3D game.
I just don't think there is any metric where someone like Birdie is less niche than Alex or Urien even if Alpha is a more popular series. Mika and Karin only have one game under their belt each too though I'm sure Ono's favoritism and polling data helped those characters respectively. I also feel Nash was added for story reasons before anything else (and if so then you can make a good argument for Urien).
Expect a skin to deal with that. Wise Zangief protecting Russian Seas.
Alternate skins just aren't the same and don't mean as much as default character designs or redesigns. You can have all the alts you want, at the end of the day the game (and most of the advertising) will treat the default design as the only one.