Medieval music theorists were attempting to link the scales used in plainchant to the ancient Greek modes. To help give the veneer of a link, and they associated modes withadopted the names of Greek regions used earlier by Aristoxenus, as the scales were somewhat analogous. The same thing would happen in the Renaissance, when theorists would again borrow the Greek names in order to establish a veneer of succession from ancient Greece.
For more, see the Wikipedia entry on Dorian Mode: Medieval Dorian Mode., and also Musical system of ancient Greece: The system of Aristoxenus