RST, used by Sphinx, isn't quite as easy to write or to read. There are plenty of gotchas. RST adds friction.
Using Sphinx also requires you to setup and configure Sphinx for your project - even more friction early on.
The world is far better for having Markdown, and it's the right choice for most projects.
However, there is indeed a place for more complicated projects to take on the workload of a more complicated documentation system so that they can help user and developers better. Django's amazing documentation is an example of this, and a major reason for the success of the project.