The devices are cheap (for what they are), the ebooks are cheap and most people are already neck deep in their ecosystem so being locked down isn't an issue to them.

Also, it's not like other ebook readers let you buy non-DRM versions of whatever books you like. Most popular ebooks have DRM of some kind.

Because most users don't care. They give Amazon a few bucks a month and they get unlimited ebooks.

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.

They're sold through Amazon.

I am using because of ease to use with Amazon. For my $8 book purchases I read once I do not want to see too much trouble.

Because they're affordable. Many people forego the idealism in favor of practicalism.

IIRC e-ink used to not be that cheap and Amazon sold you one for not that much money.

I bought a Kindle Scribe recently because it was around $250 secondhand. That makes it the cheapest of all of the 10" e-readers available. It's perfect if you want to read textbooks, PDFs, and manga.

Household name + most people don't even know about ebook DRM so they never encounter that downside + it's the books that come with DRM; you can move pirated books to it just fine.

It's usually up to the publisher whether their ebooks are DRMed or not, the Kobo store (eg) uses DRM also.

You can, of course, read non-DRMed eBooks on a Kindle, if you can find them.