Why might historians say that the Roman Empire was dismembered rather than it fell

It was the western part of the Roman Empire which fell/was dismembered. The term fall was coined by Edward Gibbon a historian who wrote the monumental work “The History of the Decline and Fall of the Roman Empire”, in the 18th century. The expression has stuck ever since.

Other historians prefer to say that the western part of the Roman Empire was dismembered because if was carved by the Germanic invaders (the Vandals. Sueves, Alans, Burundians and Alemanni) and by former Germanic allies of the Romans (the Visigoths and the Franks) who created their own kingdoms in former lands of this part of the empire. They also found the term fall inadequate as a description for the complexities of the process of the demise of the western part of the Roman Empire

The eastern part of the Roman Empire was not affected by these invasions and continued to exist for another 1,000 years. Historians have coined the term Byzantine Empire to indicate the eastern part of the Roman Empire after the fall of the western part.