Comment by noirscape

It's because of Google Play Services and the subsequent years long gutting of AOSPs application list. Google enforces OEMs to bundle their entire suite if they want to ship with Play Services and then uses that as an excuse to kill the basic AOSP phone apps that are outside of their ecosystem. That in turn harms the ability for Custom ROMs to ship basic apps like SMS readers, phone dialers and similar such, all of which used to be (maybe some still are, but they're all very much not well maintained) part of AOSP. If there's any silver lining, the upcoming play service blockade to install your own software on Android is likely going to at least create a burst of users for older devices.

The actual numbers are frankly even more disappointing - these numbers are heavily pushed up up by waydroid, which is an emulator for Android apps on Desktop Computing. More than half of the US installs is running Waydroid, with the actual most used real device in the US being nx_tab... which is LineageOS for the Nintendo Switch. That's a difference of 180k and 12k btw. The most used actual phone in the US for LineageOS is beyondx, which is the codename for the Galaxy S10 5g, a device that at a glance stopped being sold last year.

China by contrast fares much better when it comes to LineageOS (as Google Play Services isn't allowed in the country by export controls, the control from Google isn't nearly as strong there); the most used device there is actually a phone, the Xiaomi Mi 8 (dipper) and right behind it, the Xiaomi Mi 10T(/Pro) codenamed apollon.

Final country worth mentioning is Brazil, which apparently really likes the moto g7 power (ocean), a phone from roughly the same period as the Galaxy S10 5g.

Vietnam is also relatively high, with the Galaxy S7 (herolte) being the most used device. Russia is just a case where it's basically all waydroid users - not real phones.

At least from my understanding of the world, most of these numbers make sense if you consider them in proximity to US power, financial capabilities making phones last longer than their official support dates and just a rough idea of what phone brands are popular in which country.