If this game wasn't called bg 3 it wouldn't be selling like it it.
Rouge trader hit #2 for a day or two and went away. It'll be a mid success and make some money from the 40k name. But other crogs just don't sell as much and are risky.
I don't know, I think the BG branding on top of it did help with the hype because of BG2's reputation and existing fanbase. The D&D aspect is definitely a bigger deal though.
Ultimately I think it sold well because it's a really good game that was easy to turn into fun memes and clips. Plus early access meant there were a ton of people who already played it which helped the hype even more.