it takes a lot of make a character. Rendering the model is probably the smallest part of it.
Animations, balance (how much damage they do, how much damage they take, rate of fire, aoe effects, all kinds of stuff), bugs, glitches, etc.
If EA figured they can only get 8 characters ready by launch, then it's 8 characters. Fitting "2 more" in by launch isn't a trivial thing. If the 2 characters are on the roadmap, then they'll be ready later.
Sure, you could question why they're announcing them now, but it's a way to show they have a plan for future content.