Because they were from Virginia? Wasn't all that long from the admission of Texas into the Union until the start of the Civil War, was it?
John Bell Hood's Brigade was made up of Texans from Texas.
Even General Lee didn't fight for the Confederacy (at least at first) as he considered Virginia to be "his country".
This fierce loyalty to one's state is why it was called the Confederacy and not the "Southern Union".
OBTW: Many in Texas did not want to secede including Gov. Sam Houston.