Why were the Filipinos living in the distant mountain of luzon were never subjagated by the Spaniards?

The rugged terrain and dense forests of the mountainous regions in Luzon made it difficult for the Spaniards to access and control the area effectively. Additionally, the Filipinos living in these remote areas were able to adapt and resist Spanish colonization through guerrilla warfare tactics and maintaining their independence. As a result, the Spaniards focused more on establishing control over more accessible and strategic lowland areas.