Why did the south oppose the wilmot proviso bill in congress?

The South opposed the Wilmot Proviso because it would have prevented any new territories from having slavery in them.

In 1846, Congressman David Wilmot of Pennsylvania introduced the proviso as an amendment to an appropriations bill in connection with the peace treaty being negotiated with Mexico.  His amendment stipulated that any territory gained from Mexico would be free, not allowing slavery.  The specific language of the proviso went like this:
Provided, That, as an express and fundamental condition to the acquisition of any territory from the Republic of Mexico by the United States, by virtue of any treaty which may be negotiated between them, and to the use by the Executive of the moneys herein appropriated, neither slavery nor involuntary servitude shall ever exist in any part of said territory, except for crime, whereof the party shall first be duly convicted.

Wilmot's amendment passed in the House of Representatives, but was unable to get approval in the Senate.  The high-intensity debate over slavery and the expansion of slavery was evidenced by how things went with the "Wilmot Proviso."