At least in the US and Canada, the primary driver is cost. The AR-15 is extremely cheap to make because the receiver is not a pressure-bearing component, and because you need very few machines to make the most important parts (of which there are fewer than most other guns due to the design).
It doesn't help that import restrictions in the US make everything that's not an AR artificially expensive (complying with them has parts/labor overhead if it's even possible in the first place).
The other guns you mentioned are not only more expensive to produce, but they also perform much worse (the SA80 had problems with literally every single part breaking or warping, the G36 melts because its barrel is cast into plastic, the AUG fires when it shouldn't and doesn't handle as well as an AR) to the point where nearly every country that currently uses these weapons are replacing them with some variety of AR15. The original design (gas piston behind bolt = no rotational forces against the carrier = no reinforcement needed in receiver) really is that good.