Why did Genghis Khan launch a final campaign against Xi Xia in 1226?

Genghis Khan launched his final campaign against the Xi Xia kingdom in 1226 because the Xi Xia king had refused to provide military assistance to the Mongols during their expedition against Khwārezm.1

Prior to this, Genghis Khan had determined that he would not commit his main forces to the conquest of China until he had first neutralized the wealthy Tangut state of Xi Xia. The resulting campaign was highly destructive, and the Xi Xia culture was virtually annihilated by the time of Genghis Khan's death in 1227.231