I don't know. I mean, I was kinda thinking, "Dang! This game is pretty extreme with all its crazy, off the wall stuff going on," but after pouring back over the Second Sundering events, and knowing that Gorion's Ward was one of the first events of the Second Sundering, with Bhaal being reborn, and then lots of gods warring and Chosen and so on and so forth, this game's setting is set only 5 years after the Second Sundering ended.
And, to top it all off, Elturel just had the Descent happen. So, frankly, like I said, I've got to give Larian some credit here. They're working with a story that is already full of topsy-turvy turmoil with gods and such having just been apart of some pretty major conflicts just 5 years before. You may say that it's up to the writers to decide the scope, but when the scope has already been set to extreme insanity, prior to the events of this game, it's kinda hard to then slam on the breaks and not continue with the insanity.
And then, to top it all off, again, you've got to tie it back into events that took place 120 years prior? It was 1368 when Gorion's Ward and the Bhaalspawn events occurred. This game takes place in 1492. So a CRAP ton of chaos occurred between then. To top it all off, Bhaal, Bane, Myrkul, and many others have just been reborn. Duke Ravengard just took over in Gorion's Ward's place as Grand Duke. He just returned from the Descent - from being in Avernus.
Yeah, I'd say it's the setting more than the writers.
And think about it. BG1 and 2 were about a demi-god striving to either embrace or reject his/her demi-god heritage. In the end, they had a choice whether to become a god/goddess or not.
Fast forward a hundred and twenty years, gods and goddesses were just reborn and warring with each other. What do you expect from a BG sequel in terms of story? Should they not create a story about people who are gifted with some strange new powers who might eventually attain godhood? How would you create a sequel without regurgitating the originals?