Why was the Wilmot Proviso not passed?

The Wilmot Provisio passed in the House of Representatives, because the amount of representatives per state depends on the population. The population of all of the northern states was greater than that of the southern states. This means that since there were more Northern Representatives, it was passed. However, the amount of senators per state is the same regardless of population. In some sense, the power between the North and South was much more equal in the Senate as opposed to the House. That is why the Wilmot Provisio was passed in the House, but not the Senate.