A number of reasons; off the top of my head:
1. Slavery (the most important). Texas was settled by slave-owning gringos, which made its annexation relatively simple, and desirable to the Southern planter class who had the nation by the balls back then. Mexico, on the other hand, abolished slavery in 1829. Trying to integrate it into the US--which was strictly divided between slave-owning and non-slave owning states would have been a political nightmare, which not even the most avaricious Southern politician wanted to touch.
2. Military. We won the Mexican-American War through swiftness and audacity i.e. Winfield Scott's march on Mexico City (probably the greatest American feat of arms before the Vicksburg Campaign), and that was primarily done by a small professional force and temporary volunteers. We could bring the Mexican government to its knees, but we simply lacked the military power or the political will to maintain a long-term presence in Mexico; this would have cost an enormous amount of money; always an important factor.
3. The Mexican people, who--justly or not--were largely seen as ungovernable, even by their own leaders. "In a hundred years to come my people will not be fit for liberty. They do not know what it is….Despotism is the proper government for them."--Santa Anna. It would be too much work trying to integrate them--with their popish religion and their strange language--into the United States.
Any attempt to permanently conquer Mexico by the US would have eventually ended like the French occupation of Mexico.
Note: these are just a few reasons. There are a bunch more, and keep in mind that we carved off a massive chunk of Mexico as it was.
"
thanks for your reply breh