Why did various groups of people from the US migrate to Texas after it became a state?

In 1845, Texas became the 28th state to join the United States.  The statehood of Texas was quickly followed by the Mexican-American War, which did not encourage migration to the new state.  Migration continued after the war ended.

Prior to the statehood of Texas, the Mexican government enticed Americans living primarily in the south with the promise of free land.  They offered many acres of land for both farming and raising cattle.  Many of these new residents stayed in Texas when it became a state.

In the early 1840s, a German society called the Adelsverein sought to create a new Germany within Texas.  The plan was to accomplish this through mass immigration of Germans to the area.  German-Americans from U.S. states also migrated to Texas.  Ultimately, the plan to establish a New Germany was unsuccessful, though a significant German population remained.

Cotton became a key crop in Texas.  Residents from other U.S. states moved to Texas to farm cotton.  Many slaves were also brought to Texas for the purpose of growing and harvesting cotton.