Of course the AR is popular. It's easy to use, reliable, and many players will choose to invest SP in it before considering a second weapon to skill into simply because of its simplicity and familiarity. You said it yourself, you're complaining about overuse and not imbalance. From my perspective, you're also wildly exaggerating.

Expecting every gun to be represented equally among the player base is pretty ridiculous. If I ran a poll of everyone's favorite ice cream, animal, cartoon show, book, superhero, whatever, not one of these would receive equal representation throughout. Why should random chance or even balanced weaponry somehow result in balanced diversity in weapon usage?

Less than half of my deaths have been the result of lone AR fire. Hell, if I'm going to escape from any fire fight, it will be vs an AR at mid range. I'm just as likely to be under fire from scrambler rifles and in my experience someone in my LOS is probably holding a mass driver or a flaylock. Snipers are abundant. Scouts usually have a shotgun, smg, and/or flaylock. Heavies are obviously likely to be carrying HMGs or forge guns. I don't see plasma cannons, maybe one a day and that guy's probably just trying it out. Don't see many lasers. I see them, but not many.

People use the AR because it is familiar. Not OP. if it was op nobody would use the scrambler rifle.