with a focus on causation what are some of the explanations for the growing weakness of the byzantine empire in the centuries before 1450

The decay of the Byzantine Empire can be traced back to the mid 11th century when the empire was facing a triple crisis: the advance of Seljuk Empire (Turks) in the East; the advance of the Catholic Crusaders from the west; and the progressive feudalization of the Byzantine Empire.

During the following centuries, the Byzantine Empire would be under constant threat from the Seljuk Empire in Anatolia, and it also would be submerged into the conflicts of the Crusades. Several times the Crusaders from the Holy Roman Empire and from the Italian Republics wanted to take the Byzantine capital, Constantinople, as a way to extend the dominion of the Catholic Church to the territory of the Greek Orthodox Church. Indeed, the crusaders succeeded in doing so in 1204 and established the Latin Empire of Constantinople, which fell in 1261, and the Byzantine Empire was reestablished, but much weaker than before. Since then the empire would focus more on Europe, forgetting the threat coming from Asia: the Seljuks and the Mongols.

During the following century the Byzantine Empire would become one of the numerous and weak Balcanic states. At this time, one of the Seljuk sultanates that was growing in power and extension in Anatolia, the Ottoman Sultanate, became the largest threat and conquered most of what was left from the Byzantine Empire. At 1450 Constantinople was abandoned to its luck from other Christian kingdoms and ready to fall into the Ottoman hands.