Historiography is not the same thing as personal opinions of posters.
There's a difference between "What has historians said about X?" (definitely fact) and "Why did Y happen?" (possibly opinion). The former is a question of historiography, and factual, because historians write books and publish articles. Their opinions and conclusions are verifiable facts that can be cited and referenced.
The latter can be answered as a historiography, but usually is not. It tends to be approached as "why do YOU think Y happened", and when you pick one of "several different interpretations based on available historical data", that is exactly what primarily opinion based is. Citing "historical data" doesn't mean an answer wasn't opinion based; in your example specifically, most answers were simply making assertions without really backing them up.
Having said that, a question like that can usually be easily reworded into a historiography one. For example, "Why do historians think atomic weapons were used to Japan?" is a historiography question. An answer will be able to cite historians to discuss what their views are. And it probably answers the original asker's question too.
The problem is that without specifying a focus on historiography. many people tend to write answers that reads like little more than their personal opinions.
Personally I might vote to close a "why...?" question based on how its written, and vote to reopen if it is edited to be actually about historiography.