Being more feature complete might be a good reason to to support it, but it shouldn't preclude supporting what people know.
And it's easy to have different definitions of 'failed'; very few new projects use RST and Markdown has become nearly universal with a massive ecosystem around it that is utterly lacking for RST. The only places I can think of that use RST as a default have been using it for a very long time, and there would be a big cost to change.
