The Byzantines themselves ?
The various Islamic Turkish Facions?
The Roman Catholic Church?, the Orthadox Christian Church?, the schism of the Church?
The general apathy in Western Europe?
Weak Byzantine Army, reliant on mercs?
Weak for the most part Byzantine political leadership?
Poorly performing Byzantine economy?
The theological war of the Iconoclasts?
The burden of Imperial Roman ancestry?
The theory that the death of one epoch spanning empire(Byzantium) spawns another(Ottoman Turkey)?
All of the above?
None of the above?
some of the above?
Id be interested and humbled to consider your thoughts.