If the Emancipation Proclamation was issued by Abraham Lincoln in 1863,
why was it not honored in Texas?

Two reasons:

1. Politically, Texans we’re aligned with what would become the Confederate States. Even if some did want to end slavery, the state saw keeping uniform policy between southern states (all of which believed that slavery must be preserved) as the best possible scenario.

2. Much more simply, Texas had an agrarian economy reliant on cheap labor. Obviously, the cheapest labor is free, so it would be in your best interest to ignore morals and use the institution that allows such labor.