Ricardolindo
Banned
In our timeline, New Mexico had a larger Spanish settlement than the rest of the Mexican Cession. The Rio Grande provided a good transport there. In addition, unlike in the rest of the cession, there were the sedentary Pueblo, who could serve as labor. There was also a Spanish governor of New Mexico in the late 18th century who was able to militarily force a treaty on the Comanche that lasted for a couple of generations and made the Spanish population there grow and outnumber the Pueblo.
In addition, the Rio Grande also served as a good water source, very differently from Arizona, which was pretty much uninhabitable, at the time.