Why did slaveholders oppose the wilmot proviso?

Slaveholders opposed the Wilmot Proviso because it sought to ban slavery in territories acquired from Mexico, threatening the balance of power between free and slave states. They believed that prohibiting slavery in these new territories would undermine their economic interests and political influence. Additionally, many slaveholders viewed the expansion of slavery as a fundamental right and integral to their way of life, seeing the Proviso as an attack on their livelihood and social order.