During that era, the states in America was divided between Free states (where slavery is not allowed ) and slave states (Where Slavery allowed)
Back then , Texas and Florida got a lot of plantations that's heavily depended on slave , so if they were acquired , they would be part of slave states.
This will destroy the balances between slaves and and free states in the congress , ( the slave states congress will have the majority power), so the statehood was delayed