Honestly, the US and Canada are more about getting phones that are popular.
I'd say the average person in NA is less-tech-savvy than most others and that being judged for what phone you have is just part of the equation and the other part is simply sticking with something familiar. This can be influenced by marketing campaigns on TV and billboards, which Apple is, probably, #1 in the US when it comes to the iPhone. Conversely, I don't see many, if any, Mac ads. Samsung is second in presence and I honestly think that's the end of the line as far as which brands advertise on such a large scale. I don't think the average person knows Sony makes phones or what OnePlus is. A distant third, in my opinion, in advertising is Motorola. I'll see a few ads for them here and there but since they make mid level phones and low-end phones these days, it's usually on a subsidiary carrier and not one of the big three ones. It'll be on Boost Mobile or something like that, on plans for people that may not have the credit to get a post-paid plan and contract. So that influences things, too.
noticed and was annoyed by how a carrier in Canada will advertise iPhones and the newest Galaxy phones and offer virtually no choice apart from those and like 3 shitty phones nobody even looks at.
Probably to drive up subscriber numbers. If you don't let everyone know that you too have the latest and greatest from the two most popular brands, they'll assume you don't carry it and go elsewhere. "We have the Pixel 6/Moto/Xperia!" isn't going to have people breaking down TMobile's doors to get their hands on one and all these carriers are in a pretty tight race for subscribers. The only other metric they can tout is either the breadth of their 5G coverage or their 5G speeds.
I think it helps that the iPhone and Galaxy flagships do everything pretty well. So if they're popular, people don't mind the difference between cameras of the Pixel/Galaxy/iPhone/Xperia. I think most people who use their cameraphones for a living would be more inclined to pick up an iPhone rather than take a shot on the Xperia. Same with the Galaxies. I'm no professional photographer, or anything for that matter, and I would still be far more likely to stick to a Galaxy or iPhone as my next phone because it's what's popular. I don't know what Sony's track record is for supporting a phone with updates 2 or 3 years from its release and it doesn't look like a risk I would take to find out.
Aside from a pro photographer, I can't see a carrier's campaign to advertise the superior Sony camera working on other customers to entice them to buy a Sony phone. The average customer might be interested, at first, with "the best camera on a phone on the market" but even then I think they'll think they don't need pro quality pictures and would instead stick with the familiar iPhone or Samsung device. That's just my feeling. It sounds like you and I both agree that the Sony would be a real niche product for people who would be brave enough to try out a Sony phone with the best camera instead of sticking with the mainstream choices of Apple and Samsung.