I’m a crpg fan, and a D&D/PF fan. For me, the thing that makes this game so fun is it feels like a streamlined D&D session. Sure, you can’t do as much as you would like in a D&D session, but you can do 99% of what you would typically want to do.
The other thing is the game is extremely polished. So many recent games have been underproduced, unpolished garbage with DLC/MTX shoved in and a $70 price tag. BG3 is a breath of fresh air. It’s not perfect, but the care and dedication that went into it clearly shows.
I feel what makes this game so popular is the fact that the game is just really well made. The story is great, the classes are much better balanced than 5e, and the amount of interesting solutions you can use to solve any problem is just fun. Add co-op, and the game becomes a blast to play with friends.
Considering the recent rise in trrpg popularity and fans of older titles in the franchise, Larian’s existing fans, and an early access that showed off the game as being fun and promising, I’m not surprised it ended up attracting a lot of players. If you have a large enough player base at launch, and an amazing game, I don’t think it is a surprise the game is lighting the world on fire.