The only reason it took 9 years for TX to be annexed was bc of northern opposition (ironically, there were threats from Massachusetts to secede if TX became part of the US!). The reason TX whites wanted independence from Mexico in the first place was bc Mexico abolished slavery in 1829.
Mexico abolished slavery by an act of Congress in 1837 (except for forced labor of natives). Texas was already an independent Republic by 1836 Treaties of Velasco.
1829 was the Guerrero Decree, which specifically exempted Tejas y Coahuila and was irrelevant since he was deposed a few weeks later.
One major reason was slavery, yeah, not the biggest one though. The biggest reason was Santa Ana's incompetence and many many Mexican states revolted against his dictatorial takeover.
Even though white US settlers had taken an oath to obey Mexico's constitution and laws to get the land they settled on, as soon as Mexico made it clear they intended to enforce the law, even in Texas, slaveowners started talking revolution and annexation to the US.