Why did the United States complete the Gadsben Purchase in 1853

In 1853, the Mexican government kicked Americans out of the territory ... Congress ratified a revised version of the treaty; the U. S. would purchase just over 29 thousand square miles of land in exchange for $10 million. The Gadsden Purchase secured area for the transcontinental railroad and set the U. S.-Mexican border.

The land acquired in the purchase was necessary for the completion of the transcontinental railroad.

Mexico was sick of losing in battle to the US and figured it was better to get paid than fight.