MOBAS, coercive behavior and popular trends
There's nothing quite like playing for the first time a game of League of Legends or DOTA if you might. The objective of the game consists in taking down the enemy base part by part, in order to destroy the enemy core building. Teams are assembled out of 5 people, where collaboration between each other and positioning is key.
It's not particularly fun, or it stands out as other visually impressive games. It doesn't offers inmediatly complexity, variety to the objectives, the map, or even the characters (as most of them have inherently similar abilities at times). The large amount of time it takes a match to develop, as well as the fact it can not continue regularly without one of the 10 players involved would all seem to add as a detractor for newer players who'd want to get involved.
Yet it stands out as the most popular genre of today, with over 2 million concurrent players on LoL, and 500,000 on DOTA2. It doesn't seems evident for an outsider (or even an insider at times) how this could have happened. Games like Quake 3 have come and gone, without the repercusion and massivity that a game like League can offer. (Edward Zhao throws some info on the percentage of the userbase distributed among mobas that may be of use). So the question remains, why then, why this genre, why now.
"Well, they are free to play"
One of the focal point to the popularity of the genre seems to be a rather obvious one: anyone with an internet connection does can download the game and have at it. Most of the games of the genre (except SMITE) are optimized and targeted towards lower end computers, thus enabling people from all over the globe a smooth experience on the performance of the game.
The dynamics of the genre would also point to friend building and in the particular case of League of Legends, a grinding system that keeps the player working towards a goal (unlocking characters)