Many years ago, I read in a book that removing shoes in homes was common in Greece. That seemed unsurprising at the time, because I knew, being a Classics student, that the ancient Greeks removed their sandals when going indoors. However, if this book was not mistaken, it would seem that removing shoes must only be customary in parts of Greece (perhaps in the north?). All the evidence I have read indicates that Greeks generally do not care to remove their shoes in homes.
That Greeks keep their shoes on shows similarity to other southern European countries such as Italy and Spain. It does contrast very strongly with other countries in the Balkans, where shoes are always removed and often replaced with slippers or sandals. From Slovenia to Bulgaria and from Albania to Romania, shoes are removed at the door.
That Greece differs from other Balkan countries reflects the historic contrast in the Balkans between the Slavs and Hellenes. The Balkans has been divided (Illyrian Albanians excepted) between the earthly Slavs and their close connection to the land, and the more cosmopolitan Greeks, with their preference for commerce as a way of life. Where the difference in shoe etiquette came from, I do not know.
It is puzzling why Greece and Bulgaria, both Balkan, both Orthodox Christian, both at the southeast of Europe and both experiencing long years of Ottoman rule would differ over the customn of removing shoes. Is it a Slav/ Hellene difference (if so when did this difference come in)? Or is it because Bulgaria (and other Slavic countries) was under Communism and Greece was not?