Texas School shooting 19 children and 2 adults murdered

The reason it's the most popularly sold rifle in the US isn't because of fashion, it's because it is a genuinely excellent rifle for any form of civilian use. There is no need to ascribe a 'motive' beyond the rationality of the consumer. The Hyundai Tuscon isn't the most popularly sold car in Ireland because it's fashionable, it's because when a punter is looking to put his hard-earned money down, it's a sensible choice overall. If you have, say, $800 to spend on a rifle, what better choice is there to make?

The US has traditionally had former military rifles cascade down from the military to civilian use simply because it's what people know from their own service, going back to the 19th century. 30.06 as the 'standard' deer caliber goes back to the WW1 1903 Springfield and the WW2 semi-auto M1 Garand. The most common personally owned pistol for decades was the Colt 45 M1911 and its derivatives. Similarly the M-16 from which the AR-15 was derived has been the military's standard rifle for over a half-century, millions of Americans are comfortable with it, know how to use it and how to maintain it. Ammunition is cheap and easily available. It has proven reliability. 60 years of development means that there is a massive support infrastructure. It's very affordable, light enough, comfortable enough and controllable enough for anyone to use it, adjustable enough that one rifle will fit anyone in a household regardless of size difference, it's modular enough that you can use one rifle for cheap plinking with a .22 conversion, keep the 5.56 for defensive use or pest control, then swap out the top for a bigger round and you can use it to hunt deer or hog. They're not uncommon in marksmanship contests. Frankly, if you're going to own only one rifle, or a first rifle, the AR-15 has much to recommend it for pretty much any firearm use except clay pigeon shooting and duck hunting.