The short version:

• Game is good, came out at the right time, had a lot of hype and lived up to the hype

Longer details:

• The game is just really well made. It's extremely fun, very polished (except for a few weird bugs), and complete
• It has a massive IP tied to it. This game had impossible levels of hype and it met those expectations somehow
• The recent D&D movie was a large success, and D&D in general has been the most popular it has ever been lately
• Divinity OS 2 Definitive Edition was very well received, people trust Larian to deliver a good product
• People are sharing this game with their friends. They had a strong marketing push as well as really strong word of mouth
• Final Fantasy 16 left a lot of us wanting a more traditional RPG after FF16 was anything but traditional
• We currently live in an era of games like Diablo 4 which ask for a $70 price tag, and then also have a paid battle pass and paid cosmetics. This game came out at $60 content complete with no additional microtransactions. Ultimately that makes this game much easier to reccomend to people.