Really great conversation here. I think of all the job postings I've ever seen out there for a developer I only ran across one where it asked specifically for a person who was familiar with Jekyll. And I'm guessing this is one out of hundreds of jobs.
As others have pointed out, Jekyll or static site generation for that matter is more of a tool known to developers. Which is why services around Jekyll or static are targeted to the developer market instead of the consumer market.
The problem that I've seen is this though and which is why you all love Jekyll but do not love Wordpress and Drupal. Because from a developer stand point it makes it pretty easy to hack a Jekyll template with Liquid instead of getting into the nity gritty of a PHP stack. But from a consumer stand point, if you ask yourself who are the main creators of the content generated on a website, it is non-technical folks.
Those are the real end-users of a CMS and they need an interface to makes their lives easier to simply publish and get information out into the world. I think the better approach to having a marriage of Jekyll within a job place is understanding a companies' needs and seeing if Jekyll can provide a solution.
Advantages being things like static files so you can put it up on a CDN quickly and have it distribute quickly. Or low technical debt, since you're not dealing with a complex stack, with many plugins to get something working the way you want it, it is easier to maintain.
Then you can suggest Jekyll and marry it with service like Forestry.io by @Scott-Forestry.io and his team. Where as a developer you can be happy with your Jekyll projects while the end-user the content creators have a CMS-esque interface to publish without knowing how to make git commits or run builds for Jekyll.
Another way to look at it is that might be more applicable to jobs that are similar to Jekyll is looking into Shopify run companies. Small business on Shopify might want to tap into a developer that has the know how to use Liquid, which is the same templating tech for Jekyll. If you're pretty good with templating with Liquid, I'm pretty sure with a little knowledge you could pick up Shopify dev.
So another approach to the why aren't there Jekyll jobs, is get a job that is hiring for things like front-end development and then create the "Jekyll" job by introducing Jekyll to the toolbelt of tech used to build based on a company's needs.
Hope this helps. Cheers.