Well it's not just us, I mean in fairness, they have become the most popular genre in the entire gaming market. Vastly more popular even than the genres from which they were taken (RTS, RPG, and Arena Style Multiplayer).
There's just something intangibly appealing about them, and especially comparing their designs and strengths and weaknesses to one another. It's just like football or some other sport that hundreds of millions of people love. It's the elegant simplicity but the subtle depth and the individual player skill that really makes it so fun to play and watch.
At its most basic level, anybody can play football, it's extremely simple. Yet you can spend tens of thousands of hours practicing, training, discussing and theorycrafting strategies and counter-strategies to defeat your opponent, and still only scratch the surface of what's possible. Just like MOBAs, football is in a constantly shifting metagame that's always really exciting to discuss and analyze. When a team comes up with a strategy that "breaks" the metagame, or introduces a strategy that the opponent doesn't expect, it can throw the opposing team so off-kilter that they will lose to inferior players with a better overall strategy. At the same time, superior players with a worse strategy can also overcome defeat.
In the end, ARTS games offer something that few other games can. A game like Starcraft 2 may be extremely fun to watch, but there is nothing "simple" about it. You're not just controlling one unit, you're controlling often hundreds at a time which all have intricate interactions and require ungodly amounts of micromanagement. FPS games are okay to watch, but they're a bit too simple, and are usually more about team-coordination and reflexes than strategy; they don't really benefit from a "top-down" spectator perspective like ARTS games do. As I said, it's the perfect combination of the genres that give it the intangible appeal.
