Why did Byzantium (Eastern Rome) not inherit the infantry tactics of the Roman Empire? Why was the turtle formation phased out? When was it phased out?

Times have changed, cavalry is becoming more and more important, once the cavalry did not have stirrups, just increase the speed of movement, archery and fighting on the horse is difficult, not as good as the infantry, after riding roundabout to dismount to fight, with stirrups directly on the horse can fight, some opponents and even the horse also have cloak and armor, and the Roman short sword plus shield against such cavalry is very difficult.

Once upon a time, the cornerstone of the Roman legions was a strong professional soldier, their discipline, toughness, morale and skills are unique, the Eastern Roman period has not been the basis for this, the basis for semi-professional soldiers, there are also a large number of temporary conscripts and mercenaries, and at this time on the battlefield is the most important is the cavalry, the infantry is more like a mobile fortress, look at the mighty, moving too slowly to hit the people, the limited resources! Naturally, the limited resources are tilted towards the cavalry which can decide the victory or defeat.

The real meaning of plate armor appeared more in the 15th century, which is the reason of metal smelting and forging technology.

The Crusaders' fight against Byzantium was a bit like the Zhao brothers' fight against the Northern Han in the early Song Dynasty. Although the Northern Han was very tenacious, and the quality of the city was very good, but it kept losing blood, and it was slowly grinding to a halt, and before the Crusaders besieged the city, a lot of territory had been lost, and Christendom failed to provide support, and the fall was almost inevitable.