Actually Congress debated keeping all of Mexico. It was decided to allow Mexicans to have what is present day Mexico and keep only what is now the Southwest for the following reasons:
Language barrier
Culture very different than US population (worry about assimilation)
Widespread Poverty
Civilian unrest (people in Mexico had been having one revolution after another before the Mexican war)
We simply discarded the parts we did not want, it seemed it would be more trouble than it was worth to keep all of Mexico. At the time it was a wise choice, but if they continue to be bad neighbors we could always reconsider. Retaking Mexico would not take much effort.
They've injured our country enough with the infusion of drugs and criminal dregs that paying us for it with their oil might become a reasonable outcome...