Because half the people working at Bioware were terrible retards who chose to give their side-studio their next installment, using a game engine that couldn't do RPG mechanics to make an RPG, then decided to make it a procedurally generated RPG about exploring a representation of an entire galaxy in an engine that couldn't do procedural generation, so they had to build all of that from the ground up, only for it to not work, oh yeah, and EA probably forced them to use Frostbite. So they had to scrap the entire game and build some dogshit in 18 months, of which they spent probably a third making terrible homosex scenes of the clay golems they call characters. Now, if they were less shit at their jobs, or had less women hired, they probably could've made a better game, since FNV was also made in 18 months, but hey. So yeah, I can think of a few reasons.