We need caution in order to understand what happened. All the reasons provided by the film (most of them to be found in many history books) participate but are not enough to explain the Fall.
Byzantium existed in the middle of attacks, obliged in a constant defence. The makers of the film of course know this, but they skip it saying that “the empire had experienced all these things before, and had overcome them.” What they fail to understand is that each time Byzantium ‘overcame’ a problem, a serious wound was left in its body. The examples are many, if we want to know them. Justinian managed to regain the western provinces defeating the barbarians, in order to leave Byzantium exhausted and ready to lose the provinces later; resistance against the Persians left Byzantium unable to resist the Arab invasion, etc.
Nowhere in the film is made a remark about this, although it is obvious and has been emphasized many times by many historians, that Byzantium in the course of time was getting almost identical with Constantinople, since provinces were being lost one by one, and even Constantinople to the Crusaders for half a century. However, I can not blame the makers of the film for such mistakes, when I remember that even some Byzantines, foolish men like Pletho, believed that certain political and economical changes would save them against the Ottomans. The emperors tried hard and with sacrifices to get the only feasible help, western help, which, as the directors of the film also note, was never provided.
Although having developed a science of war, as Runciman observes, Byzantium was not a military state. Obliged in a constant war, no matter how powerful it might have been, no matter how well it knew to combat, obviously it could not resist a global attack – from the East, the South, the North, and even from the West, from the Christian nations. That acknowledged, it is no wonder that Byzantium fell, but that it resisted for 11 centuries.
Beyond these and above all explanations one might give, we can not understand Byzantium if we imagine it to be a ‘nation’. With every province that was lost, especially of the western provinces that signified the start of the empire in the older Rome, Byzantines received not only a political wound, but even more seriously a wound in their soul, in their ecumenical nature. A Byzantium that was getting identified with Greeks, due to the loss of eastern and western provinces, was not losing just power, but its very nature.