I think a lot of it comes down to the presensation.
I can't really talk about the depth of the game because I haven't played DOS2 so I can't compare, but BG3 on top of being insane with its depth, also offers a lot of things that make it more attractive for people like me who usually aren't into these CRPG games.
For example, I really dislike top down / isometric camera. When a game uses a camera like this, I'm instantly a lot less interested, because the immersion is really not the same and I don't see myself enjoying exploration when it's presented like that. Baldur's Gate 3 lets you zoom in the camera and play it with a more traditional third person camera, making the game feel a lot more like games like Dragon Age Origins for example, which I liked too. I still do use the zoomed out camera a lot, I change the distance all the time depending on what I do and what I'm looking at, but having the option to see it from a more immersive perspective is a huge bonus for me.
From what I've seen in Divinity, the dialogues are much better here too. I don't mean the content of the dialogue, but one again, the presensation. The pacing seems better, everything has cutscene, fully voiced, with facial animations, enough content to be interested but not too much to get tedious, it's a better presensation for me that the lot of text you can get in other games of the genre.
This game just feels like a much larger Dragon Age Origins, with elements of the "Immersive Sim" genre added to it, and that's really great.
I also like that I'm able to control my character directly with wasd instead of only clicks, but that's thanks to a mod someone made, which works really great, and I don't understand why the devs didn't allow this by default on mouse & keyboard setup when it's available for controllers, really weird choice. Having both the ergonomics of mouse & keyboard + also access to the wasd controls make it the best way to play it for me.