[coliveira]

The only reason for this is that power changed from Romans to tribes in Northern Europe. If you trace genealogies, you'll see that modern families descend from tribal leaders in Northern Europe, and these ancient kings had nothing to do with the old power elites of Rome. Of course, the thing is not so simple, because with the help of Catholic Church there were some marriages arranged between the so-called barbarian rulers and noble families in Rome. So the powerful Roman families still survive in some way, but not in a direct lineage like it was before the fall of Rome.