Why did president polk think the mexican government might want to sell california and new mexico?

"Polk is often considered the last strong pre–Civil War president, having met during his four years in the office every major domestic and foreign policy goal set during his campaign and the transition to his administration. When Mexico rejected the annexation of Texas by the United States, Polk achieved a sweeping victory in the Mexican–American War, which resulted in the cession by Mexico of nearly the whole of what is now the American Southwest. He ensured a substantial reduction of tariff rates by replacing the "Black Tariff" with the Walker tariff of 1846, which pleased the less-industrialized states of his native South by rendering less expensive both imported and, through competition, domestic goods. He threatened war with the United Kingdom over the issue of which nation owned the Oregon Country, eventually reaching a settlement in which the British were made to sell the portion that became the Oregon Territory. In addition, he re-established the Independent Treasury System (supplanted by the Federal Reserve System in 1913), oversaw the opening of the United States Naval Academy and the Smithsonian Institution, the groundbreaking for the Washington Monument, and the issuance of the first United States postage stamp. Scholars have ranked him favorably on lists of greatest presidents for his ability to promote, obtain support for, and achieve all of the major items on his presidential agenda. However, he has also been criticized for leading the country into war against Mexico and for exacerbating sectional divides. Polk has been called the "least known consequential president".