As an outsider to the genre myself
@Dynamo Frokane, this is only from what I have observed and experienced from those who hold it in high, passionate regard.
League of Legends, as well as similar games for that matter, really capitalize on short, brief, high intensity conflict in a scenario that has a lot of meta. There is a lot of optimization, study and analysis that goes into actually playing "decently". It is exceptionally competitive because of that. Not only does a player need have rapid, snap-like reactions, they need dexterity; no less, strategy and coordination are pretty key. A well executed, coordinated team is going to run through competition. That means having a social circle of like-minded who all have their own talent in the same medium.
And once you have those people, you have peers and friends, with whom you are likely sharing a lot of personality and interests with. But the game itself?
It is diverse, with the aforementioned meta, statistics and play styles. You could be horrible at a number of characters, but when you find that one niche that leads to success, you can imagine players finally feel successful. All the frustrations of the community and other players fades in large part when they fall away.
In addition, I suppose it is needing mention, but the game is forgiving. You might lose a few games, but play with a few people you know and suddenly you find so much less issue. Until of course you find those who are just generally better at the game and with a slight favor of advantage like the factors of luck or those equally unpredictable.
At least that is
my theory.
