The appeal would be with a limited albeit large set of characters, items, & rules, you can have effectively an infinite set of outcomes due to the dice rolls of teammates but also champions/heroes chosen on team. It is almost impossible to see the same game twice unlike. There is skill expression & build mechanics that allow a player to outplay or recover matchups & adjust to the state of the game on the fly. With every game starting over at zero, you don’t get invested in building a specific character, but in mastering the gameplay which can go from micro mechanics to macro. I think a lot of folks liked it coincidentally at a time with better broadband for communications for this style of game, developers doing frequent patches to force meta shakeups & e-sports + streaming also taking off. But also a sunk cost fallacy of having invested the time to git gud not bothering to learn any game too similar.