I'm confused. Mead isn't beer. How does the BJCP come into play when judging mead?
Oh, you ask a very simple question, with a rather complex answer there!
But let me try to give you the summary answer. Essentially, they do because 1) Nobody else was doing it when the BJCP came on the scene, and 2) There historically was a lot of overlap between home beer brewers and home meadmakers early in the development of the hobby here in the US (that would be roughly in the period between 1977 and 1997). To many beer brewers of that era, meadmaking was a logical extension of their hobby, so they "adopted" the meadmaking community and provided a framework around which some standard mead making processes and mead judging criteria could be established.
Many of the seasoned veterans here on Gotmead have made as much beer as mead, and several are BJCP certified beer judges already. It is to the credit of folks like Ken Schramm, Pete Bakulic (Oskaar), Vicky, Mike Zapolski (Hightest) and several others that the guidelines for judging mead have been significantly developed, refined and enhanced (not to mention corrected) over the past decade, and it is in large part because of their efforts that a separate mead judge designation exists within the current BJCP framework.
That said, there may now exist enough independent interest in meadmaking to support organization of a mead-centric education and evaluation entity... but the amount of work required to pull off something like that is daunting, and gaining acceptance (or at least avoiding overt criticism) from the BJCP and other beer brewers may not be easy.