The Comanches raided settlements to capture women and children because of their low fertility rate. Quanah fought to save his mother, Cythia Ann Parker, from the Rangers. The Comanches gave up due to the diminishing levels of buffalo (because they got most of what they needed from them) caused by the mass hunting of buffalo being used as a tactic against the Comanches.(sorry if I'm overexplaining just filling in gaps that might help a small bit)
My guess would be because they wanted to be discreet so not to cause an uprising or have people opinons be over riding their thinking
The US was the biggest imperializer in Latin America