Final answer:
Henry Clay lost the 1844 presidential election due to his association with policies that were out of step with the expansionist sentiment of the era, as well as a strategic party decision to prioritize candidates with military success who could appeal to a broader electorate.
Step-by-step explanation:
In 1844, Whig Party candidate Henry Clay lost the presidential election largely because of his association with policies that did not align with the expansionist sentiment of the time. His commitment to the American System and his status as a Kentucky slaveholder were seen as liabilities by a younger generation of Whig politicians, who focused on finding a candidate that could represent the common man. Moreover, the Whigs' decision to bypass Clay in favor of war heroes like William Henry Harrison and Zachary Taylor also reflected the party's recognition of the popularity of military success as a selling point to voters.
Despite his influence and leadership, Henry Clay's political positions and the Whigs' strategy to maintain party unity by not adopting any platform ultimately worked against him. In the context of territorial expansion and the Mexican-American War, his opposition to such policies made him less desirable as a national leader during a period of American enthusiasm for westward growth. The Whigs needed a candidate like General Zachary Taylor, who was a war hero, and had a broad appeal, especially after the Mexican Cession.