I'm mystified. I've been on the platform less than two years, have published just under 300 stories, and in the past view months, my earnings have shot up significantly. I don't have a niche. I write all kinds of stuff. My guess is that the algorithms that used to benefit people like you who have lots of followers and have made a lot more money than 99% of us have shifted. This might be because Medium attracted a lot of new writers with the MWC and they want to encourage them to stay. It's just a guess. Good luck with your book.