Why did Mexico want to stop Americans from settling in Texas after 1824?

Tejas, in English history books usually referred to as Mexican Texas, was a province of Mexico between 1821 and 1836. Mexico gained independence from Spain in 1821 in its war of independence. Initially, Mexican Texas operated very similarly to Spanish Texas. However, the 1824 Constitution of Mexico set up a federal structure, with Tejas joined with the province of Coahuila to form the state of Coahuila y Tejas.

Tejas was grossly underpopulated, with about 3500 settlers living in the whole of Tejas in 1821, mostly congregated at San Antonio and La Bahia,[1] despite efforts by the authorities to increase the settler population along the frontier. The settler population was overwhelmingly outnumbered by the indigenous tribes. To increase settler numbers, Mexico enacted the General Colonization Law in 1824, which enabled all heads of household, regardless of race, religion or immigrant status, to land in Mexico. The first empresarial grant had been made under Spanish control to Stephen F. Austin, whose settlers, known as the Old Three Hundred, settled along the Brazos River in 1822. The grant was later ratified by the Mexican government. Twenty-three other empresarios brought settlers to the state, the majority from the United States of America, while others came from Mexico and Europe.

After concerns over attitudes of US citizens in Tejas, the Law of April 6, 1830 outlawed further immigration of US citizens to Texas. Several new presidios were established in the region to monitor immigration and customs practices. Angry colonists held a convention in 1832 to demand that US citizens be allowed to immigrate. A convention the following year proposed that Texas become a separate Mexican state. Although Mexico implemented several measures to appease the colonists, Antonio Lopez de Santa Anna's measures to transform Mexico from a federalist to a centralist state motivated the Texan colonists to revolt.