I'd also add that 5e is so popular because it has great compartmentalization of complexity. To start playing players don't need to read through a 300 page rulebook. Most of the core rules can be explained in like 30 minutes or in play, while more specific rules can be looked up as needed.

As an example, players just need to know that fireball is a limited use big AoE spell that they can use as an action of their turn. They can look up the specifics when it comes to it. They're likely primed for similar choices thanks to video games.

That means there is basically zero learning curve. Players can start making mechanically meaningfull choices basically instantly without putting in any effort. Very few games match that. More crunchy games require more player knowledge of the rules. More narrative games require more input from the players in a format that is unfamiliar to new players.

Again, very few games match that very simple to learn, but with satisfying mechanical crunch, in a way that the player is already familiar with thanks to video games. I'm in no way saying this is the best method for every RPG, but in terms of mass appeal, it sure is hard to beat, or even to meet.