End customers don't even know that Nexus/Pixel phones exist (because you mostly cannot buy them everywhere) and they don't know the benefit from such devices and they also don't care deeply enough.

The way people buy phones is a lot more complicated. Most people pick a carrier first, and then the phone; and most people buy their phone from their carrier store (or a carrier booth at a place like Costco). Thus the consumer choices are controlled by the carrier. If a carrier doesn't want to sell a Pixel phone, then few will be sold, regardless of what consumers might 'want'.

Apple is obviously an exception to this due to their own advertising and stores.

I haven't looked at the numbers, but they're unavailable in a lot of countries, and are kind of expensive since a few generations.

They can't become popular if they're difficult to buy.

Even Google abandons their products after a couple of releases. Perhaps better supported than other manufacturers but I'm running 7.1.2 on a Nexus 4 only through Lineage OS.

They might be, if people actually knew that they existed...