Amazon's ecosystem. Every major publisher is on the kindle store and it's pretty much the first place any indie author self-publishes to. Kindle Unlimited has a pretty good selection of stuff for a flat subscription and (at least in the US) you can borrow e-books from your local library through libby and access them on the kindle.

The simplicity of it makes it really appealing to users who just want to read books and not think too much about it. Android e-readers also tend to have far worse battery life and higher-friction integration with the ecosystem.