The main reason for this was slavery. The US did not want to annex Texas because doing so would have upset the balance between slave states and free states that had been accomplished with the Missouri Compromise of 1820.
When Texas became independent, it wanted to join up with the United States. However, many people in the North felt that the Southern slaveholders had cooked up a plot to get Texas to rebel so as to get another slave state into the Union. In order to prevent a sectional crisis over slavery, President Jackson and subsequent presidents did not annex Texas until 1845.
When Texas got its independence from Mexico in 1836, it wanted to join the United States as a state. However, Texas didn’t become a state until December 29, 1845. There were a few reasons for this long delay.
One reason centered on the issue of slavery. There had been a balance between free states and slave states in our country. Northerners were against the annexation of Texas because it would have given the South another slave state. The Northerners were concerned this would allow for pro-slavery policies and laws to be developed and passed in our country.
There was also some concern that annexation Texas immediately could have led to war with Mexico. The Mexican government didn’t officially recognize Texas as an independent nation. Santa Anna agreed to give Texas its freedom in exchange for his release, but the full government didn’t officially acknowledge the status of Texas as an independent country. Thus, there was some concern that the annexation of Texas could possibly lead to a war with Mexico.
As a result, Texas was an independent republic, called The Lone Star Republic, from 1836-1845.