I think there are a couple of issues. One is simply development time; they had certain characters in mind and that was all they had the ability to develop in the available time. I know I wouldn't want EA to go on too much longer. I also think it's a repercussion of going with an undersized party. By limiting party size to four they've made it less rewarding to have more characters. You're already forced to leave more characters on the bench than you can bring with you with only 10 total companions (9 of whom you can have on your bench in a given playthrough). Adding two or three more characters would just make everything that much harder.
