That's not to say it's not a good rifle though, but I don't think that in a military context it's fundamentally better or worse than any of its peers, such as the H&K G36, the Steyr AUG, or the SA80.
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.
At which point is one weapon merely a variant of another? Isn't this change in the action a significant improvement?
[1] https://en.wikipedia.org/wiki/M27_Infantry_Automatic_Rifle
In fact, many such piston AR retrofits exist in the US civilian AR market, but they’re generally thought to be of “unproven” longevity, at least.
The advantages of the retrofit piston being slim and light is that the recoil increase is small. The disadvantage is that durability is questionable, as those pistons experience a lot of violent force and most guns designed from scratch to be piston operated have significantly heavier duty pistons, for good reason.
You can read online of Marine armorers lamenting how their M27s are breaking down at a rate much faster than their M4s. This is of course anecdotal, but it is also not entirely worthless data.
The "direct impingement" system of the M16 / AR15 isn't really true direct impingement, as the bolt itself works as a gas piston within the bolt carrier. But the gas has to travel much further to reach it than with a conventional piston.
The AR 15 platform has benefited hugely from a lot of refinement and investment over a very long time. That's certainly true, but Stoner's concept for it is just really, really good and the fact that the design issues were all tractable and the further refinements have been largely successful is testament to that fundamental soundness.
Or the FAMAS (or maybe it's just my Gallic pride talking ;))