ReST can be better extended without creating a variant of the markup language [1]. Extension support is first class [2], and it is by that means that a good number of the core features are implemented.

[1] For example, github flavour, stackoverflow flavour, reddit flavour, ...

[2] Roles and directives.

One large difference is that Markdown exclusively targets HTML as an output format where ReST tries to be format agnostic. If you only ever target HTML this might ease some things but not if you want html, man pages, pdfs, plain text, …

ReST is a whole multi-document system, as opposed to MD which is just the formatting (at least before you mix in the dozens of mutually incompatible extensions). ReST is a superset.