There are certain genres of which 90% of authors are publishing exclusively on Amazon, because all other authors in that genre are too and that's where the customer base is. They all publish under Kindle Unlimited, which allows readers with a subscription to read unlimited books (of which authors get paid a certain % of that months total KU profit).

That phenomena isn't a coincidence, a large percentage of readers in this genre will have KU because almost the entire genre they love is on there. To sign up for KU requires authors to agree to an exclusive publishing agreement, meaning they can't publish their ebooks on other platforms. This means that the majority of that genre will never migrate to other platforms.