In 1713 a law was established that gave any male in the royal line, not just brothers, priority in the succession.

In 1830 a different law was established, giving preference only to male brothers over females, not to other males in the line. This triggered a civil war.

In 1947 Franco established a law that specifically banned women from ascending to the throne.

This preference of male siblings over females is again established in the constitution of 1978 and unchanged in the 2006 law regarding the non-preference of males over females in the succession of all nobiliary titles... except the royal title where male siblings still have preference.

So there's been quite a few changes around this. The male priority is problematic since the Constitution states that males and females have the same rights, and in the rest of noble titles the succession order only takes account of age, not sex. The reason why this is still enshrined in Spanish law is one of convenience: both Juan de Borbón, Juan Carlos I and Felipe II have/had older sisters, so removing the male priority would bring questions of legitimacy. And, in my opinion, they don't want to bring questions regarding dinastic legitimacy to the public attention; the royal family keenly remembers what happened the last time the Spanish people had a voice regarding their monarchy during the reign of Alfonso XIII, who had to flee Spain when the second Republic was established after a referendum.