I think it's the simplicity. Guns are very direct mechanisms and you often are only balancing a few resources in your head (health vs. ammunition), if at all. There are some complicating factors around mobility, cover, teamwork (in... most games these days) but they're all very intuitive when it's all about people holding, pointing and shooting at each other. Combined with well-designed maps, the gameplay flow (even in multiplayer) can be quite tightly crafted to have a very compelling pacing, leading to a very compelling experience. First-person shooters, by virtue of being first-person, tend to be the most immersive, giving them the biggest draw.
In addition to this, shooters are easily augmented with additive addictive factors, such as random item drops, experience gains and progression, etc. which have become practically ubiquitous in modern online shooters. Going beyond game mechanics, the core gamer demographic are males who statistically are interested in explosions, guns, and such things, so it becomes a bit of a self-reinforcing thing- developers make 'safe' games for the core demographic, which in turn makes it more likely to convert non-gamers from that core demographic rather than outside of it.