I'd say the big reason is that Sphinx (restructuredText) has a syntax geared towards documentation (info/warning boxes, etc) so we didn't have to adopt a custom markdown flavor to achieve the same results. We also have the option to easily build other variants such as PDFs, man pages, texinfo, etc should we choose. In a perfect world, I would prefer if there was a "standard" markdown flavor that existed for documentation that mimics the key directives of the RST syntax. The markdown ecosystem is getting there, but until then sphinx works for us.
We currently use Jekyll (possibly Gatsby in the future) for www.netgate.com and www.pfsense.org so we are familiar with those SSGs as well.