-French North American mainland territory had been unproductive and costly to maintain in past wars, something that Napoleon knew he could not afford in the upcoming wars.
-Britain had already declared war on France, surely part of the reason was to try and encourage better Franco-American relations. Surely Napoleon had some idea of American pride and stubbornness as well, once the U.S. controlled the territory
-Napoleon knew that there was no way any European power (Britain specifically) would conquer the territory.
Napoleon needed money and the sale of the Louisiana territory brought France some much needed capital.