I'd imagine the main reason for Birthsigns removal(more like re-purpose) is twofold. First of all, the actual "Birthsign" itself, is now just what your character is, instead of more these zodiac-like boons and curses. Every character will have a Birthsign, and there's technically more than a million potential ones, at least in spirit and theory, I guess they figured at that point, it didn't make sense to keep the "Zodiac" birthsigns.
Secondly, but probably least influential in the decision, was, most birthsigns resulted in arbitrary attribute bonuses, and few were particularly balanced well.
Now, I'm not definding the removal of the birthsigns, personally, I liked the amount of cultural flare they added to the world. It was like a common thread that everyone had in Tamriel, but I am not going to get up-in-arms over the loss. I personally think the loss of the titled Birthsigns is a bigger hit to Roleplaying than the removal of attributes ever will be.