Logistics. Even if they had "won" in 1836, they couldn't have kept an army in Texas for long. No food, no supplies, and no port. The reason Texas was always poorly defended, from the Spanish times to Mexican times, was they didn't have enough people to sustain an army. When Arredondo conquered San Antonio in 1813, and kept his army in Texas for a year, he basically starved the colony. It was one of the reasons (along with his brutality) that accelerated the downward spiral of Hispanic Texas.
Even North Korea doesn't have a military-to-civilian ratio comparable to what he had, and what Mexico would have had to have had, to hold Texas. Assuming 100 percent of Anglos and 25-50 percent of Hispanics were hostile, they would have needed an extensive, and vulnerable supply line just to keep their army from starving.