Because the juramentado moro was unstoppable.
Before attacking anyone, the kris-wielding moro would take lots of opium, making almost his entire body numb and unable to feel any pain. Bullets from an ordinary handgun, e.g., a .38 calibre revolver would merely penetrate his body but he would continue his attack to reach his intended victim.
It was because of this experience during the Filipino-American war in 1899 that the Americans invented the .45 cal. pistol to stop the charging moro before he could reach the American soldier.
The United States claimed the territories of the Philippines after the Spanish–American War. The ethnic Moro population of the southern Philippines resisted both Spanish and United States colonization. The Spaniards were restricted to a handful of coastal garrisons or Forts and they made occasional punitive expeditions into the vast interior regions. (Source: Wikipedia)
Following the American occupation of the Northern Philippines, Spanish forces in the Southern Philippines were abolished, and they retreated to the garrisons at Zamboanga and Jolo. American forces took control over the Spanish government in Jolo and Zamboanga in December 1899. (ibid)