Haven't played Divinity 2, tried a couple of times but something just turns me off, so don't know too much about the system even though it doesn't appeal particularly to me, but that aside it seems like a pretty huge undertaking to take up in a fully voice acted game. Somewhat wasteful too, all things considered. I mean, i think even from the most casual of casual players perspective the appeal of playing a premade character or a custom one must be around the same. And i very much doubt that kind of crowd would even ever play the story as different origins.
A lot of people like to play predefined characters. Such characters usually have a better connection to the game world. Games like witcher and me are popular for a reason. In the case of custom characters, writing a story of their own for them is difficult because it would require them to have some kind of "frame". It would be nice to have a prology like in DAO, but some people will complain that the game takes away their freedom.