I think BG3 is riding on the D&D brand and marketing campaign

With how much they adapted 5e's rules for a video game (thankfully, 5e can be jank) I'd wager it's more to do with them riding on the Forgotten Realms setting. It's hugely popular (see: BG 1-2, dozens of books, most popular D&D setting through the last few editions) so it helps drive interest that there's a competent game that is both faithful to the lore and excels at storytelling.

For instance I really liked a tiny scrap of paper dealing with the Mines of Phandelver hundreds of years ago. That's a bit of flavor from the 5e Red Box. Tons of stuff like that calling back to adventures and books in the series.