* Texas was a sovereign nation. When Texas declared independence from Mexico in 1836, it became a sovereign nation. The United States did not want to annex a sovereign nation without its consent.
* There was opposition to annexation in the United States. Many people in the United States opposed the annexation of Texas. Some people feared that annexing Texas would lead to war with Mexico. Others feared that annexing Texas would upset the balance of power between the North and the South.
* The Mexican-American War. The United States and Mexico went to war over the annexation of Texas. The war lasted from 1846 to 1848 and resulted in the United States gaining control of Texas and other territories in the Southwest.
The combination of these factors made it take a long time for the United States to annex Texas.