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The French system of laws is a civil law system that is based upon the Napoleonic Code, and further back the Justinian Code. The legal system in Louisiana is now bijuridicial retaining some of the elements of the Napoleonic Code.
But, why did the rest of the land that was purchased as part of the Louisiana Purchase not adopt a civil law system, like Louisiana did? All of the other states that were originally part of the Louisiana Purchase now have common law systems.
Today's Louisiana, with its port of New Orleans, was the part of the Louisiana Territory that was most developed and populated when it was sold to America in 1803. Hence, the state was largely entrenched in its French ways. The LATER settlement by English speakers from the rest of the U.S. created a "bijudicial" system that retained a heavy French influence.
On the other hand, the other 12 states or so of the Purchase were basically empty. The whole purpose of the Lewis and Clark expedition was to explore most of these states (other than Louisiana). They were later settled by Americans used to English common law. But there was (practically) no French influence beforehand.
