The problem with Cardassians and the Breen is, they are more known by fans, not people that know Star Trek.
Romulans are, Klingons are. That's why they were there (Klingons only in the simulation of the Maru, but still), to appeal to the casual watchers of Star Trek and people that have never seen an episode.
If you're going to stuff the next film full of characters and races that only the fans would know, you're making things less appealing to non-fans. And face it, they are marketing these films to everyone, not just hardcore fans. They used to do that, and what happened? Trek films attracted less and less viewers in the cinema's, because Trek fans are only a small portion of all cinema-visiting viewers. And in the end of the day, it's all about one thing: money. Sure, Abhrams and co are into Trek themselves, but they need to make money, that's what the studios want.
So, bringing in T'Pring could still work, since there was only a little backstory. But bringing in the Cardies is a whole other level. Trekkers will understand everything about them. The casual fan, or even non-fan who was attracted by the trailer and internetbuzz, will not. You need to explain things to much, which costs screentime and slows down plots.
Making it simple, by either using established races or introducing complete new once that don't need deep exploring, or can be explored throughout the plot/film, makes things more interesting to everyone, not just hardcore fans like you and me.