These are devices built with a specific use case in mind that need an operating system that is easy to develop for. Non-Google versions of Android have struggled to make much headway in markets like smartphones. Even Amazon's own phone flopped pretty hard.

> Non-Google versions of Android have struggled to make much headway in markets like smartphones

You may have forgotten about China: Huawei smartphones are doing very well on an AOSP-based OS.

Edit: Google was concerned about a Chinese fork of Android flourishing, they wrote to congress at the time the tech-transfer ban was being condidered

The thing about Android is that a device doesn't have to make headway by selling 100 million units plus to continue being adapted to many different types of devices.

In any case, the Play store can be sideloaded on every Android device even if it isn't officially supported.

>Non-Google versions of Android have struggled to make much headway in markets like smartphones. Even Amazon's own phone flopped pretty hard.

Not much headway? Pretty much all Android phones used in China do not come with GMS.

Kindle tablets on the other hand have done well.