I think you've got an extra 0 in there.Some reasons for the AR15 and it's many variants is simply due to the fact that they work as they should, quite accurate, easily assembled by most shooters to any configuration desired, economical to shoot versus other calibers, low recoil, and the world is full of aftermarket items to fit the rifle. It is estimated that there are more than 150,000,000 of such rifles in the hands of the general population. There may well be, but I know of no other firearm that popular.
To me, the AR-15 is the Harley Davidson of firearm's. You can "make it yours", just like a Harley. And making this possible, the "Evo powerplant" made it "easy for the copycats to jump on the band wagon"!1. Through military service, M16 was the very first firearms experience of any kind for many, many people over the last 50 or so years. The "modern sporting rifle" is completely normal to them. How many people under 30 today would have any idea how to use a rotary phone, or a typewriter?
2. As Stalin famously said, "Quantity has a quality all its own." When something works well at an affordable price point and gains a certain level of popularity, the popularity will snowball as aftermarket goodies arrive and more popularity ensues, which breeds more mods and accessories, and so on and so on. We've seen this in the firearms world with the 1911 and the 10/22, just to name two, and in the car world with the Ford flathead V8, the small-block Chevy, the VW Beetle, and the 5.0 Mustang, to name a few. I'm sure there are examples in other realms, too.
When I went looking for my first "assault rifle" around 1985 or '86, the AR was at the bottom of my list. I'd done six years with them as a USAF small arms instructor, so they had no exotic allure for me; I thought they had no "soul." The Galil and HK93 and Valmet 76 were at the top of my list, with the FNC as a dark horse candidate. But the Galil was heavy and didn't seem to have the accuracy I was looking for, and I couldn't find a Valmet to even handle or examine (or an FNC). The HK93 fit me perfectly and actually seemed to whisper "Buy me!" in my ear, and then I asked about extra magazines and was quoted an absolutely eye-watering price. Didn't want any of THAT. The AR goodies boom hadn't quite taken off yet, but I knew you could hardly walk through a gun show without having to kick cheap surplus M16 mags out of your way. I ended up with a nice one-owner Colt SP-1 (with the original Colt 3X scope, no less!) at a reasonable (not cheap) price, and I'm glad I did. And I still have it.
This is a huge reason, How many of us like Magpul? The slim line stuff made a noticeable difference on mine and it feels more like it was made for me. I still like the more mainstream Magpul MOE furniture also. There dozens of end plates to fit to your way to sling it as well as on the buttstocks and there are dozens upon dozens of those, I just got a minimalist buttstock and really like it too. Tons of different kinds of charging handles to fit you. My favorite aftermarket part is a nice 3 Lb self contained trigger. Plus better performing parts are coming out all the time.and the world is full of aftermarket items to fit the rifle.