- 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.
Many people will claim "graphics don't matter", but the reality is that they do help.
PS: One thing that is a lot better than in BG1 and BG2 is that combat with low-class characters feels a lot more interesting. Not sure how much of this is because of the ADnD vs DnD5 rules or whether Larian has added some tweaks to the DnD5 rules.