Ancient genomes aren't spread evenly across the region. Some areas and periods are extremely well sampled (Cretan Bronze Age, Bulgarian Eneolithic, Croatian Late Antiquity), while others are thin (Cypriot Iron Age, Albanian Neolithic, Bosnian everything). If we built the breakdown purely from ancient DNA, the well-sampled zones would dominate and the under-sampled ones would give you shaky, unstable results.
So instead we use modern geographic and cultural names as stable, clear anchors that map onto the ancient DNA landscape — and then interpret each anchor using time-resolved ancient DNA transects, archaeological context, and published reference panels. That way you get both fine geographic detail and real depth in time.