One of the main reasons why Congress did not initially annex Texas is because it would create an imbalance of free and slave states in Congress.
During the early to mid 19th century, the United States was constantly expanding their territory. This was due in large part to the concept of manifest destiny. As the US was spreading their influence, they ran into the problem of whether or not these new territories and states would have the institution of slavery.
Southern states favored new territories and states having slavery while Northern states did not favor slavery in these new territories. The reason behind the Northern states includeds:
1) Influences from the abolitionist movement.
2) Northerners did not want slave states to have more representatives in Congress than they did.
This caused Texas annexation to be delayed, since Texas would be admitted as a slave state. This would throw off the balance of free and slave states represented in Congress, giving slave states the advantage.
