To play devil's advocate in this case and to be honest, Spain has the distinction of having kinda pathetic history from Napoleon's era until 1970s so it's not like we lose major player of 19th and 20th centuries who played a major role in great wars and adventures of those eras

That may be the reason why devs didn't bother with modern Spain and focused on its glorious medieval and early modern eras - and we have many better slot uses than Castille.
At least that's what I took out from reading about the trainwreck of Spanish political history post Napoleon, its impotence against former colonies, inability to industrialize or get onto the train of new colonialism, insignificance in global affairs, civil war and then obscurantism of theocratic Franco regime. At several points in 20th century Spain had lower gdp per capita than several Eastern Europen countries and even its own colonies. And I'm not even talking about Czechoslovakia and Argentina but at one point
Poland and
Mexico lol.
On the other hand, I have always respected Spanish ability to somehow emerge as the "normal" Western country with high economic, political and social standards - after all this mediocrity and misery.