The short answer is that AR-15s are just better rifles. They're more accurate, they're more reliable, they're easier to clean and maintain, they're easier to repair, they have much better ergonomics, none of the parts are proprietary, and consequently there's an enormous aftermarket for parts, accessories, and customization. They also have a modular design that, with the exception of the barrel nut and castle nut which have torque specifications, can be almost completely disassembled with a single roll punch and an allen wrench or two. That means if something breaks or wears out you don't have to send it back to the manufacturer or pay out the nose for a gunsmith, you can just order the part and fix it yourself with basically just a pointy stick and a YouTube video. It also means you can start out with a really cheap rifle and upgrade it component by component until you have a high-end rifle if you want to.

That Mini-14 on the bottom is a fine rifle, and they're actually pretty popular, but the AR platform outclasses it on most crucial metrics. If you could only have one or the other, for most people it'd be the AR without question. A lot of people have spilled a lot of ink speculating about this reason or that reason as to why so many people want ARs, and usually manage to miss the fact that they're just fantastic rifles. Even with the amount of cringey fetishizing of the military that happens on the conservative side of the gun community, nobody would want one if they sucked.