>>713178809
It was one of the first to really explode in popularity. Another thing is the massive amount of items that are in the game and the way that they can stack. The result is that almost every run feels unique from getting some insane combo of items. And even people who are dogshit at the game can win just by getting a lucky item combo. A shit ton of unlocks also means that there will often be a goal for the player.
The best way to summarize is that boi is really good at the stuff that makes roguelikes addictive to people. Shit's basically crack.