Why did Texas secede from the US in 1861?Texans spent 15 years trying to gain admission to the Union, yet despite Sam Houston’s objections and...

If you follow the utk.edu link below, you can read the
"Declaration of the Causes which Impel the State of Texas to Secede from the Federal
Union."  This will tell you, in the words of the Texan secessionists themselves, why
Texas chose to secede from the Union.  From the words of the declaration, it is quite
clear that slavery and the desire to maintain the slave system was the major cause of
Texas's secession from the Union.

As you read the
declaration, you should be struck by how many times the text refers to slavery.  Slavery
is the first thing mentioned in the substance of the declaration and it is the last. 
The text mentions at the start that Texas came into the Union as a slave state and it
says that the federal government has been unfair to the slave states.  At the end of the
declaration, they state that they hold as an "undeniable truth"
that

...the
servitude of the African race, as existing in these States, is mutually beneficial to
both bond and free, and is abundantly authorized and justified by the experience of
mankind, and the revealed will of the Almighty Creator, as recognized by all Christian
nations; while the destruction of the existing relations between the two races, as
advocated by our sectional enemies, would bring inevitable calamities upon both and
desolation upon the fifteen slave-holding
states.

From this, it is
quite clear that the desire to maintain the slave system was Texas's main reason for
seceding.