Why was the spread of slavery into Texas a cause of concern for Mexico prior to Texas's independence?

Because Slavery is terrible and Slaves were coming form Mexico

Explanation:

The history of slavery in Texas, as a colonial territory, then part of Mexico, later Republic in 1836, and U.S. state in 1845, began slowly. The use of slavery expanded in the mid-nineteenth century as White American settlers, primarily from the Southeastern United States, crossed the Mississippi River and brought slaves with them. Slavery was present in Spanish America and Mexico prior to the arrival of American settlers, but it was not highly developed, and the Spanish did not rely on it for labor during their years in Spanish Texas.

im sorry i wish i can but try google