chapotraphouse

To be precise, while slavery was immensely profitable, continuing to fill the continent with oppressed people was a tinderbox waiting to ignite, and settlers on both sides were quite self-aware about that. It's hard to choose just one quote, but here's one from Settlers chapter 4 to explain this:

We can only understand the deep passions of the slavery dispute, the flaring gunfights in Missouri and "Bloody Kansas" between pro-slavery and anti-slavery settlers, and lastly the grinding, monumental Civil War of 1861-1865, as the final play of this greatest contradiction in the settler ranks. It was not freedom for Afrikans that motivated them. No, the reverse. It was their own futures, their own fortunes. Gov. Morton of Ohio called on his fellows to realize their true interests: "We are all personally interested in this question, not indirectly and remotely as in a mere political abstraction - but directly, pecuniarily, and selfishly. If we do not exclude slavery from the Territories, it will exclude us."

To millions of Euro-Amerikans in the North, the slave system had to be halted because it filled the land with masses of Afrikans instead of masses of settlers. To be precise: In the 19th Century a consensus emerged among the majority of Euro-Amerikans that just as the Indian nations before them, the dangerous Afrikan colony had to be at first contained and then totally eliminated, so that the land could be filled by the loyal settler citizens of the Empire.