Well rst is better than markdown from day one. The only reason it became famous is thanks to wikimarkup.
So markdown needs to thank the popularity of Wikipedia for its success, as rst did not have any application like Wikipedia. But still rst is used widely enough with its killer Sphinx, readthedocs and now its kind of de-facto documentation writing markup in Python and many open source software world.
You don't really get bit by its lack of a standard and extensibility until after you've bought in.
It's essentially designed by the opposite of a committee -- rather than including everything but the kitchen-sink, it contains support for almost no usecases except the one. Which is very appealing, when you only have the one usecase.