To get a direct answer, I think one would need to interview the textbook makers themselves. I think the best we could do here is hazard a guess. Here's mine.

A few months ago we had a Spanish Civil War AMA and I answered a similar question as follows:

I understand your concern and to a certain point I share it. However, Paul Preston points out that the Spanish Civil War has generated nearly as much scholarship as World War Two--at least in English. That's just three years of conflict confined to one country, compared to a longer timespan and a territory that spans the globe. So I also understand a teacher deciding not to complicate the issue for junior high and high school students--especially with the Spanish Civil War not being a subject that is likely to be on the current standardized tests. Given the complexity of the subject, it might be better to just leave it for undergraduates and gifted youngsters (despite my own fascination with it).

To address your question specifically, I think it is the complexity of the subject that makes studying it more daunting for anything short of undergraduate work. I mean, just answering a question about what could be attractive about fascism took both /u/tobbinator and I a few paragraphs each to take a stab at it. Even just keeping track of all of Franco's potential rivals can be a handful, not to mention the alphabet soup that was the Republican opposition. So an aversion to tackle the complexity would be my answer, but I am always open to followup questions.