Probably because it's difficult to determine the supply of developers for a language. Job boards tend to be companies posting jobs, not candidates posting (public) resumes.

Even if you had resume data, having a language listed as a proficiency doesn't mean that person is in the supply of developers for that language. Likewise, sometimes people get jobs for languages they don't know. I hadn't used golang before my current job, so I wouldn't have been in the "supply" of go developers despite being hired to use it.