- Their previous game Divinity: Original Sin 2 was critically acclaimed, very popular for a pretty hardcore CRPG, and had long legs.
- DnD has a lot of brand power and has been strongly in the zeitgeist for years.
- There's a big cohort of millennials who have strong nostalgia for Baldur's Gate and who have plenty of money to buy games (if not time to play them).
- The Early Access release for this game was wildly popular beyond the developer's expectations, and maintained interest for years.
I definitely underestimated the brand power of DnD and Baldur's Gate because they aren't very important to me, personally. But also there have been a load of really good CRPGs in recent years and there seemed to be a pretty low ceiling to how much interest they could get. Tyranny, Pillars of Eternity, Pathfinder: Kingmaker, and a few others were amazing and beloved CRPG games but were lucky to have a tenth of the success of BG3. But those games were generally less accessible, mostly not multiplayer, and again lacked the brand power.
The last D&D game I remember, Dark Alliance, is horrible.
Anyway, I was just surprised because I guess I never looked up a review of the Dark Alliance games but my general impression was actually pretty good.
edit: Apparently there is a Dark Alliance game with a naming collision that came out much more recently than the Dark Alliance series I'm thinking of. Smart move, Wizards of the Coast/DnD.