Time and resources limit branches in the main story (see the reply to "But the choices you make influence the storyline, I suppose?" in
Dragon Commander: Interview with Swen Vincke). Trying to carry the effects of multiple plot branches into sequels compounds the effect significantly. Now if Divinity had the budget of Mass Effect...
One of the few things we know about Project E (aka Divinity 3) is that we will be able to defeat the main bad guy in the end.