We all have a huge cultural load, developed trough movies, books, comics, scale models, toys...
There's also the aesthetic aspect. German machines have distinctive lines and style which has captived eyes through decades in spite of ideologies. There's also some mysticism...
About the periods, there're many Spaniards more interested in WWII than in SCW. Some of them, have still too strong feelings about SCW to consider it interesting or fun for a game.
For Spanish people, It really was as you say, David: they were mostly caught in one of the sides. For the special case of pilots, I'm reading many memories books of pilots of both sides and they all agree:
Whenever a pilot of the other side was captured, it was taken in custody by the enemy air forces, that protected him and provided all the necessary, offering him to be interchanged. This was possible because all the professional pilots had been colleagues before war, so they knew or even were friends of each other.
For instance, Garcia Morato asked Nationalist pilots to protect any Republican pilot bailing out, since the Republicans had been doing so before.
There was some sense of solidarity and gentlemen fair play between pilots.
On the battle ground, things were very different, and very strong feelings still arise nowadays for many people.
About non Spaniards feeling interested in that period, I understand them, because It's the Golden age of Aviation, look at the aircraft that we are getting: most of them have historical records or amazing history.
Besides, many foreigners can feel involved with SCW the same way that Spaniards do. This was, probably, the most ideological war of 20th century. It still awakes passionate feelings for one side or the other.
For many, It represents the fight for people's liberty and the struggle against the oppressive forces of capital and hierarchy, which is becoming kind of a current situation.
I visit Belchite very often and there are always foreigners visiting too. When Spanish people ask me why those German, Polish, Czech, French, American, Russian people come here, I usually answer them: Well, their parents or grandparents are still buried here too.
About 100,000 Italians, 60,000 Moroccans, 30,000 Germans, 20,000 Portuguese and 600 Irish fought with Franco, more than 10,000 Russians, and about 50,000 people from more than 50 countries volunteered in International Brigades or in the Anarchist and POUM millitias.
Many are still buried in Spain, many are still in the trenches were they died.