Shrewd politics and bartering with the underlying threat of war and subterfuge after centuries of proven genocide on the indigenous peoples. It was clear the US was going to eventually try to kill everyone between the East and West coasts that stood against them, and that the US Americans weren't leaving. Smart to sell.

Not unrelated, this is why the US needs to get the fuck out of the Mid-East and Afghanistan. We are not going to ever "win" there, because US settlers are not going to move to Kabul or Damascus or Baghdad. The people fighting against us there will not leave.