Not certain why, other than they changed the algorithm. Consistent part of the equations is the people. Readers aren't going to change that much. That's the market. Once you find it, you keep it entertained and the keep coming back. But, when the algorithm changes under new leadership you can go from 800 a month in earnings and 25 thousand views and readers a month downward. Change the algorithm in a way that it's slow, it chops a percentage off every month. That's what I've seen.
Then there's the reads. The algorithm isn't making much sense mathematically speaking. Medium would have one believe that known members that have been with you are reading only a few seconds of your story, clapping, and running off. What they're not taking into account in my opinion are the connections. Most people have promoted on social media and probably connected with a few people. Therefore, instead of 5 reads of a ten minute story, you'd have at least ten minutes because you know one read the entire story, and left the screen open to make sure it counted. 5 full reads should have been 23 to 25 minutes but I'll go as far as maybe 4 out of 5 don't read it all.
Membership will die off because of that. People like to write, but when you spend X amount of hours working up something that is a first page response on google, and "nobody" on Medium ever reads it, then all your doing is sharing it on someone else's site to get them noticed. People like to see some value for what they're doing, and that's a majority of them. After a while people stop doing things there's no benefit or value in doing.