When the Empire split in 2 in the year 284 AD they split the latin speaking culture west and the Greek speaking culture east. This made things difficult, since most Emperors on either side stopped supporting each other with military and trading which hurt both halves... Later on in the 5th century the west fell due to barbarian incursion and lack of military, while the greeks were prospering in the natural rich resources in the east. Also the east had better natural barriers between them and the savage barbarians.
Hope this helps :)
