Why was westward expansion such an important issue in the election of 1844?

It is thought to have been the reason why Polk won the election over Clay. Polk was a slave owner and was in favor of annexing Texas which was a slave state already, thus growing the slavery movement. Clay was initially against annexing Texas when Van Buren was initially running against him. When Polk ended up winning the nomination for the Democratic party over Van Buren, Clay then changed his mind on the topic of the annexation to support it, as Polk had. This made him appear wishy-washy and led to be Polk becoming President.