It is for a similar reason AR-15s are so popular here in the USA, because the US Military mostly uses M16s ,which are almost the same, and therefore they are the "cool, I'm almost-for-really a soldier now" gun.
- Lots of manufacturers
- Reliable action (depending on manufacturer)
- 5.56mm NATO and/or .223 Remington ammo is (or, before late 2012, was) relatively cheap
- Light recoil
- Tons of accessories available
- Relatively accurate
- Fun to shoot
And also, among gun guys, there's at least a little bit of the feeling that by owning one, you're giving the middle finger to people who want to ban them.
Saying someone owns an AR is sort of like (loosely) saying they own a PC - that could mean anything from an Intel NUC to a Xeon workstation in a huge case. There's a common architecture, but it's the flexibility of the architecture that makes it popular.
Sure, because we all know how safe guns are kept in the USA and no one ever runs amok with a few of them around.
The "tacticool" factor is probably responsible for only a tiny fraction of all AR-15 sales.