Why can't historians agree on why Rome fell?

Because there isn't one cause, there are hundreds of candidate causes, and historians keep finding new evidence that changes the ranking. One scholar, Alexander Demandt, counted 210 separate theories proposed to explain Rome's fall. Historians today weigh the army, the economy, the emperors' competence, power struggles, religious change and the climate.

The modern version of the question starts with Edward Gibbon, whose six-volume The History of the Decline and Fall of the Roman Empire, published between 1776 and 1789, quickly became the standard account. Gibbon is often remembered for blaming Christianity: in his view, wealth that would have served the state went to the Church instead. Few scholars accept that today; the switch to Christianity probably made little difference to public finances. The idea stuck in popular memory anyway.

Modern scholarship has more data than Gibbon ever had: archaeology, epidemiology, climate records and even genetics. Some of that evidence points to nature rather than morality. A long warm, stable period that had made Mediterranean farming and tax collection easy started to deteriorate after 150 AD, worsening further after 450 in what's called the Late Antique Little Ice Age. Some historians now see climate and disease as important drivers of the collapse.

Even the word 'fall' is disputed. By 476, when the Germanic king Odoacer deposed the last Western emperor, a boy nicknamed Romulus Augustulus, 'little Augustus', the emperor already controlled almost nothing real; barbarian kingdoms already ran most of the West. Some historians now prefer to call this a slow 'transformation' rather than a sudden collapse, pointing to archaeological continuity in settlement patterns lasting centuries longer. Others answer that the loss of political unity and military control is not in doubt: the western empire really did collapse, just with many interacting causes rather than a single villain.