YAML is unused as a configuration file for end users. Or at least it should be.
I'm curious who are these end users that would be scared / turned off by yaml but are totally cool with markdown.
YAML was crafted to be usable by most mere mortals. Whether or not it achieves that in practice is another issue ... but I think most people who find themselves using a static-site generator could find their way around a YAML config file just fine.