I think it has to do with the fact that Guitartricks both has to work with requests from members here and the publishing companies which allows Guitartricks to actually learn this songs legally.
So for example: If there are a lot of members who request more rock and metal songs Guitartricks will choose those first, since those have the most requests. Also it could be that country songs are harder to get with the publishing companies.
But as I understand Guitartricks tries to keep everyone as happy as possible, being open to all suggestions. They also want to appeal as wide an audience as possible so there will always be country/blues and other songs they will teach. I agree with you that there are currently few country (and blues) songs. Mostly rock and metal now.
Hope that helps clear things up.
JJ