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.
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.
