Thomas Jefferson had always feared the costs of loose construction of the powers delegated to the national government in the Constitution, and the Constitution was silent about acquiring lands from other countries. Jefferson urged bringing the issue to the people to approve with a constitutional amendment, but Congress disregarded his draft amendments. The Senate ratified the Louisiana Purchase Treaty in October of 1803. While Jefferson did his best to follow what he believed was proper constitutional procedure, not enough of his contemporaries agreed with him and he eventually assented.
I believe it was because, that was such a great victory for the colonists over the British that after it was all over, everyone had heard about it. Hope this helps.
