I'd respond to the original question, Why do people leave Mastodon?, with a different question: Why has Mastodon become the most successful decentralized platform in such a short time?
If I'm not mistaken, Mastodon now has about the same number of accounts as Diaspora, but it took just 7 months versus 3 years. The "waves", as mentioned, come and go, and it's been shown again and again that there are always more lookie-lookers than active users of any platform. Why would Mastodon be any different? This forum will be the same way. Facebook, Twitter, and Wikipedia, despite their massive numbers, have massive amounts of dead accounts. Business as usual in the sign-up world.
Point is, let's not be too concerned about what isn't a concern yet, if ever. Mastodon is clearly doing something right. Or, maybe it's a sign early adopters are getting tired of those other places and really want something different. I mean, who isn't sick of the words "Facebook" and "Twitter" rattling in the brain... Or "Zuckerberg", for that matter, and the other silicon calfs of worship. (Maybe it's just me.)
And in that vein of thinking, Mastodon should not compare itself with centralized socmed platforms on the basis of population count. Centralized and decentralized are two different models of operation. It's like comparing a skateboard with rollerblades -- they share a medium and means (road, wheels, and leg muscles), but otherwise quite different too.
Mastodon should not copy every expected feature or functional trend associated with centralized social media. Give Masto some time to let it's quick success gel into it's own direction. It may set the new trend. Who knows. But not if it's a copy-cat.
Frankly, social buttons all over a web UI sucks ass. And I know a lot of people who have left Facebook and/or Twitter, or will be soon because they are sick of those worlds. In my real existence, I work in digital communications, and social buttons are not only being seen as a bad design consideration (except by content marketers, of course; the people everyone is getting annoyed by), but they're increasingly disliked for the shameless marketing purposes they often serve, solely. And let's not even talk about the code bloat that social button injections add to websites.
Use them. Don't use them, but...
Don't fall into the trap of thinking Facebook, Twitter, Google+, etc are doing it right. Those platforms and the "tricks" they provide are designed to psychologically manipulate people.
Mastodon can be a lot better than the money-driven hucksters of Silicon Valley by showing what a ethical platform should be. Centralized socmed has the numbers right now, yes, but don't overlook the fact those sites are showing their age and problems. Twitter can't make money, and Facebook has a fake news problem.
Mastodon may only need to play the long game, not compete in the short run. Focus on being good and righteous and valuable — and the right people will come when they abandon what isn't cool anymore, and they will help guide others in turn.