I think part of it is that it's a single-player RPG (or co-op if you want) with a focus on the story experience. No mmo-lite features, battlepasses, always online requirements, live service, etc. bloat to be found. A concept that the current AAA market seems to be allergic to.

It says what it is on the tin and delivers that. That's a breath of fresh air for many people right now.

Also, there was that whole drama with devs complaining about how BG3 was going to ruin it for everybody else by making gamers expect a higher quality product from studios. Which isn't really what those devs were saying, but it's what it came off as to people, and that probably gave the game a lot more positive attention than it probably otherwise would've gotten.