Answer:
I believe it is because it was expensive, and the U.S was independent from Texas. Mexico also held the west coast, and pretty much everything was in Kaos, because Texas Independence.
Step-by-step explanation:
Mexico held what is now known as Arizona, California, New Mexico, Texas, Colorado, Nevada, and Utah, which is pretty bigg.
Mexico and Texas wasn't getting along, and the U.S was slightly supporting the U.S so that didn't go over so well.
President Houston tried to smooth things over with Polk, but Lamar kinda went banzai and attacked everything and tried to Manifest Destiny Texas instead of the U.S, and Lamar hated joining the U.S, and Mexico said that if Texas joined the U.S, "Texas + U.S = war." which kinda blew up into war.