They're the most popular pretty much by default though, it didn't happen organically. They're the most popular because GW has made them available in introductory products in every edition of the game, which is something that other factions only get for a single edition, if at all.
Like if you wanted get into AoS and you really wanted Blades of Khorne, you missed the boat to do that cheaply 6 years ago; now they're just as expensive as every other faction. Same for Nighthaunt, and it will be the same for Kruleboyz once we transition to 4E starter products next year. You get one edition to start a non-Stormcast army using more affordable starter products, and if you come to the game later you just miss out.
Exact same goes for 40k. If you want to start the game from scratch there's always an intro product for you with Space Marines in it, but if Death Guard are your thing then you better hope you arrived on the scene between the summers of 2017 and 2020 or you're out of luck.
Fantasy battle had 5 starter sets between 4th and 8th Edition that featured 4 different "good" factions. There's no reason we can't have a bit of that variety in AoS as well.