The issue was that the Louisiana territory was almost completely unihabited by Europeans, and had no large cities or governmental systems, which are necessary for forming states. The U.S. had a fairly low population, and was still 'filling' the territory east of the Mississippi with people at the time. The new land would only slowly get immigrants for a long while.

I forget exactly how many residents a territory needed to apply for statehood, somewhere around 20,000 I believe. The U.S. wanted a territory to have a sustainable population and at least basic infrastructure and local government before it became a state, and became beholden to the higher taxes and standards states had to follow.