Garbage in, garbage out. Github's language detection isn't perfect, and people in the D community simply aren't particularly focused on marketing and popularity contests. See here for example :

http://www.mail-archive.com/digitalmars-d-learn@puremagic.co...

Not only is it not quite dead yet, it's growing rapidly : http://erdani.com/d/downloads.daily.png

DMD downloads direct from home page only.