The difference is that System Shock Remake is as the name implies: a remake, of a very old game no less. Baldur's Gate 3 is a full on modern AAA game, with all the bells and whistles. It doesn't need to hide behind nostalgia, it can stand head to head with all the other big games out there. And it comes as the successor to the Divinity games which themselves already were massively popular.

Demon Souls might be better comparison here, started out as a rather niche PS3 title, build a fan base over the years, had numerous sequels and follow ups that all matched or exceed the quality, and Elden Ring is a gigantic hit now.

I guess doing quality games for a decade or more just accumulates a lot of fans and positive word of mouth, so much that even people that aren't hardcore into the genre get sucked in.