Following Justinian's megalomania the state didn't have enough forces to guard the ancient borders along the Danube, so slavs got in and settled in the region. Even during Basil II's time, or later the great Komnenoi restoration, despite the empire controlling the balkans the population was still majority slav and pretty much impossible to keep the region unless nothing major happened (eg fourth crusade).
That said, at least the hellespont + Constantinople could have been kept in 1922. Blame France for insisting all of eastern Thrace was given to Turkey - let alone vetoing the english declaring war on Turkey then.