I am a bit shocked by how popular this game is. All the signs were there, though. - 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…
I think you're trying to put too much of its success on "brand power". Social proof is a thing, but it's not powerful enough to overcome a bad game. Just look at what happened to the Call of Duty and Battlefield franchises.
I think it's doing so well because they nailed the execution. The graphics are great. The game hasn't had any opening week disasters despite getting more than 7x the expected numbers. The intro hook of the game really grabs you and keeps you wanting more (which is why everyone's still so excited about it 3 years later). Also, it's FUN to play.
That last one is probably the biggest factor. When was the last time a AAA game was truly fun to play? It wasn't too long ago that online streamers were publishing videos lamenting the lack of good games to play.