What's your source on that? What we call country coalesced in the '20s, and bluegrass followed in the '40s, not taking that name until the '50s. It's of course difficult to put dates on musical styles, what with them constantly evolving and borrowing- both styles could use lyrics taken all the way back from England. But going by when each were recorded and marketed as the distinct genres we know now, I don't know why one would put bluegrass first.