In addition to being off-topic, it's heavily biased, highlighting the (reprehensible) civilian casualties caused by the Nationalists, while remaining silent on the similar number of (also reprehensible) civilian deaths caused by the Republicans.
It also describes the Nationalists as Fascists (which is true), while describing the anarchist and communist backed Republicans as "left-leaning" (which is biased understatement).
Most likely because people found it interesting. Not everyone finds the same things interesting.
> which is biased understatement
Funny thing regarding biases in political leaning is that it's all based around context. The US, being very right-leaning, would describe one person as left-leaning while in many European countries that same person would be center or even slightly right. So it all depends. It's useful to outline where you're coming from before laying out your perspective, as it's also biased (just like mine).
Or it just highlights the complete uselessness of the right-left dichotomy.
But you know that...
But I don't think this is actually true or a useful statement. Many aspects of American politics are more left-wing than their European counterparts; anarchism or libertarianism, for example, roughly defined as antipathy toward the state, is often considered left-wing and is far more prevalent in the US. Left-wing politics in contemporary Europe seems to almost always include a strong state role.
https://en.wikipedia.org/wiki/Libertarian_anarchism
As I said, I don't think any sort of universal right-left dichotomy is useful. It also doesn't factor in non-traditional political thinkers, like say, Nietzsche, who could not be accurately described as right, left, or anywhere in between.
There is no context on which the Republican side was not comprised of communist and socialist groups supported by Stalin's Soviet Union.
That conflict was an incredibly complex and tragic series of events... beyond the scope of what an article like this can cover. It wasn't a commies vs. nazis event, and this wasn't an article doing a survey view of the conflict.
Nationalist Spain also enjoyed extreme support by both capitalist Western Europe and the Soviet Union. If your theory of equal popularity being necessary for a protracted civil war, asymmetric foreign support indicates that the non-Nationalist side would have higher social support.
P.D. The Soviet Union sided with the republicans until they unfairly appropriate (aka stole) the spanish national treasure.
You might want to do a bit of reading:
https://en.wikipedia.org/wiki/German_involvement_in_the_Span...
The Spanish Civil War soon became more about blue-on-blue infighting between traditional leftists and straight-up Stalinist agents. Orwell did not flee Spain because of the Nationalists closing in, he was being hunted by soviet agents.