Slavery was prohibited north of 36°30′ latitude by the Missouri Compromise of 1820. So Texas just gave the land north of that to Oklahoma to keep slavery legal.
The Compromise of 1850 said that no state above 36° latitude could be a slave state. At the time Texas extended all the way to modern day Wyoming. In order to remain a slave state they gave up all the land north of that. This is why Oklahoma has a narrow panhandle now.
