Explanation:
1. The U.S. had Superior Firepower
Artillery (cannons and mortars) was an important part of warfare in 1846. The Mexicans had decent artillery, including the legendary St. Patrick's Battalion, but the Americans had the best in the world at the time. American cannon crews had roughly double the effective range of their Mexican counterparts and their deadly, accurate fire made the difference in several battles, most notably the Battle of Palo Alto. Also, the Americans first deployed the "flying artillery" in this war: relatively lightweight but deadly cannons and mortars that could be swiftly redeployed to different parts of the battlefield as needed. This advance in artillery strategy greatly helped the American war effort.
2. Better Generals
The American invasion from the north was led by General Zachary Taylor, who would later become President of the United States. Taylor was an excellent strategist: when faced with the imposingly fortified city of Monterrey, he saw its weakness right away: the fortified points of the city were too far from one another: his battle plan was to pick them off one by one. The second American army, attacking from the east, was led by General Winfield Scott, probably the best tactical General of his generation. He liked to attack where he was least expected and more than once surprised his opponents by coming at them from seemingly out of nowhere. His plans for battles such as Cerro Gordo and Chapultepec were masterful. The Mexican Generals, such as the legendarily inept Antonio Lopez de Santa Anna, were way outclassed.
3. Better Resources
The American government committed plenty of cash to the war effort. The soldiers had good guns and uniforms, enough food, high-quality artillery and horses and just about everything else they needed. The Mexicans, on the other hand, were totally broke during the entire war. "Loans" were forced from the rich and the church, but still corruption was rampant and the soldiers were poorly equipped and trained. Ammunition was often in short supply: the Battle of Churubusco might have resulted in a Mexican victory, had ammunition arrived for the defenders in time.
