Because he was angry at france and England is my conclusion. And your bringing up the Louisianna purchase is perfectly appropriate in my view. It is another example of where Jefferson decided to circumvent the constitution....I believe he admitted it too didn't he? Proving that even he wasn't averse to that in certain circumstances.
In a letter to his friend and Senator Wilson Cary Nicholas (who strongly argued against the need for an Amendment to grant additional constitutional authority for the federal government to obtain land from foreign nations trough means other than war) Jefferson expressed his lingering constitutional concerns about the purchase of the Louisiana territories in this way ...
"When an instrument admits two constructions, the one safe, the other dangerous, the one precise, the other indefinite, I prefer that which is safe & precise. I had rather ask an enlargement of power from the nation where it is found necessary, than to assume it by a construction which would make our powers boundless. Our peculiar security is in possession of a written Constitution. Let us not make it a blank paper by construction."
Conceding the likelihood that the framers' enumeration of powers was "defective" -- for "this is the ordinary case of all human works" -- he urged, "Let us go on then perfecting it, by adding by way of amendment to the constitution, those powers which time & trial show are still wanting." In the present case, he concluded, it was "important . . . to set an example against broad construction by appealing for new power to the people."
But after all that, he went ahead with the purchase and put aside the Amendment that he (himself) drafted in order to fulfill his constitutional duty, for the purpose of expediency. I wish he had attempted to pass the Amendment as quickly as it could have been done. It no doubt would have passed easily, and it would have at least helped prolonged the time it took to turn the Constitution into a "boundless" piece of "blank paper".