People don't appreciate just how much work went into SFxT.
Out of 43 pre-DLC characters, 27 were brand new from scratch.
TWENTY-SEVEN.
Then, with the DLC characters, 7 out of 12 were brand new models and animations. So, overall, that's 34 completely new characters (and, it's not like all the SF4 characters didn't undergo lots of changes and gain new stuff, too).
THIRTY-FOUR.
That's a heck of a lot of brand new models and movelists, especially when compared to most other new fighting games these days.
That's why they went with Ibuki instead of yet ANOTHER brand new from scratch character. Ono already collapsed once during the creation of SFxT. That should tell you what the work load was already like. Imagine if they had to also create Sodom, Karin, and whomever else people wanted from scratch.
EDIT: Also, keep in mind that most of those brand new models and movelists belonged to TEKKEN characters. Meaning that they could never be reused in a future SF game, unlike when most 3D fighting game companyies create new animations. Tekken 7, for example, is still using decade old animations from old games. This won't be the case for most SFxT animations (heck, Capcom is already starting from scratch with SF5). Most SFxT animations were created JUST for SFxT. So, next time someone accuses Capcom of being "lazy", remind them of this.