Aerospray has always been a weapon that was common among the inexperienced. It paints well, and after the general range nerf in Splatoon 2 their lackluster range isn't as glaring as in Splatoon 1, they still aren't great.
Lot of newer players will probably commit to a single weapon of choice, Aerospray being a common one among those. And they'll take it into rank with reckless abandonment for how mediocre they may be in the mode.
Since we're in the dead of Summer, we're probably getting an influx of newer players, which means a lot more newbies that'll probably pickup the Aerospray.
The MG has a niche use for Splat Zones since its special is great for turfing zones, and the RG is a Baller machine. But other than those purposes (which I doubt you'll see people taking full advantage of in B rank), the rest is just a new wave of player picking up the weapon most likely.
Their presence of the weapon does taper off once you start getting in higher ranks. Like A onwards (or S), you'll occasionally see them, but they certainly are a lot less common place once you start climbing the ladder.