Because people didn't believe all of his theory's of Natural Selection and the revolving of species.
The ones who controlled the territories would have the main political power in the congress. If the states were slave states then it would bother non-slave states in the congress, and vice versa. That's why it was necessary to create new states that weren't the former Mexican ones that would help balance out the power of the two factions.
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The united states signed the Louisiana purchase with France.