I'm not arguing one is better, just that Markdown is simpler: it literally has less syntax. If I don't need all the bells and whistles of reST, Markdown is simpler to learn, use and likely implement.
I agree that reST is probably a better fit for complex use cases like building a docs site. But it's overkill for Facebook Messenger or even forum posts, IMO.
If you want to use a mark-up language in a chat app, a subset of either would serve you just fine and both are equally simple. It sounds like you are comparing a subset of what-is-markdown-today (and there are still variants) to full reST, which I don't think is a fair comparison.
From a very brief look, here's some things that probably aren't generally necessary (and aren't included in Markdown): Document structure, Bibliographic fields, complex footnotes and citations, directives, substitution definitions
All of these increase the number of things I have to escape, as well as my cognitive load when writing documents. And that's before we talk about reST config settings!
You could use a subset of reST, or you could just use Markdown!
I agree that reST is probably a better fit for complex use cases like building a docs site. But it's overkill for Facebook Messenger or even forum posts, IMO.