Why didn't the Americans take all of Mexico (Mexican American War)?

The US hasn't generally been inclined to take over countries against their will because a democracy by its very nature is not in a very good position to be an imperial power. Keeping the populated areas of Mexico under control, and suppressing rebellions, would've required a lot of troops. That in turn would've required conscription, even in peacetime, and increased war output at the expense of civilian or consumer goods. Naturally voters wouldn't have liked that, and would've voted for someone who promised to end our occupation. In addition, crushing rebellions would've meant casualties, and atrocities, and a free press would've reported these, further ending support for the occupation. Basically, the democratic US realized it had to limit its territorial ambitions because it wasn't cut out to be an imperial power.