It’s because most of the things in a Chaplain build will not scale with STR outside of the first swing of the basic attack. Most of the damage from all the buffs are fixed damage that doesn’t scale, with the exception of Asperglium which brings me to the next point:
As long as the skill says [magic] it scales with magic damage. Int increases magic damage so it scales with INT.
The bulk of Chaplain’s basic attack damage will be aspersion because it still gets boosted from its own attribute, so you can boost int and increase the damage of spells like Aspersion while buffing heals and other offensive spells while having meaty normal attacks.