[Qwertious]

There's no single one "what", but:

Western rome kept feuding with itself and hiring germanic tribes as mercenaries. After a while, the germanic mercenaries were thoroughly trained in fighting western romans, and eventually western rome pissed off germanic mercenaries and lost the resulting war.

Eastern rome didn't have as many germanic tribes as neighbors, and they had less infighting to begin with.

One of the major reasons earlier-rome had so much military success was that they conquered their neighbors before said neighbors successfully copied all their (really quite amazing) military doctrines.

(I am not a historian, don't quote me on this)