I have absolutely nothing to back this up, but I have a feeling that EA's greenlighting of this game was on the condition that it be open world. EA doesn't have any open world franchises, and even if Mirror's Edge doesn't sell, at least the engine work is done and can be used for future titles.
Or, maybe it was purely DICE's decision and I'm full of it. I'm just surprised we got a new Mirror's Edge game at all when the first one underperformed, and I could see EA having some say in the direction in order for it to get made.