Ease of use has been on the first place when you mention AK-47, although the AK has more recoil than, for example, an M16. Besides that, the AK has other advantages and significant disadvantages. The AK is a heavy gun (all that steel and wood may be cheap, but it’s heavy). The AK’s rounds are more powerful and more likely to kill you than the 5.56 NATO used by the M16–because they have higher kinetic energy, higher caliber, and depend less on velocity-affected yawing and tumbling effects–but the rounds also weigh more than the 5.56.
It’s cheap. The AK’s simple design–and the use of iron, steel, and wood – make it cheap. This is obviously a boon for militias and terrorist organizations with limited resources. And indeed for any non-Western army. It also helps that the Soviet Union and China gave out millions of them to whoever wanted them. Global average price for an AK-47 or derivative is $534, although they can be considerably cheaper in conflict zones where supply is plentiful.
Ease of use
It’s extremely simple to operate. Children can and have carried the AK-47 to war. The Avtomat Kalashnikov 47 is a gun that can be operated in many adverse conditions – like sand, mud, or wet jungles – with relatively little threat of jamming. (This is not to say that AKs never jam; just they don’t this as much as most other comparable weapons would in the circumstances.) The AK gets this reliability because of its simple design and high tolerances used in the construction; although the latter has consequences for accuracy. The M16, for example, is more accurate than your typical AK.
It is reliable and has been proven so over many decades. This example was found carried by a Somali pirate. Despite the fact that it is covered in rust, it still worked.