Why did the French sell the Louisiana Purchase to the USA?

Answer 1

It's a little complicated, but there were several factors:

France didn't really "own" the Louisiana territory. That is, they kind of did in theory, but much of it was fairly loosely held and not readily defensible. Spain had formerly claimed much of the territory, and for France to sell it made the Spanish claims Somebody Else's Problem.

It was also a long way off. Napoleon was far more interested in increasing his European holdings than in maintaining France's grip on a lot of wilderness of dubious (at the time) value in the New World. France had already lost Haiti, which was a considerably more lucrative (again, at the time) piece of real estate.

Finally, selling it to the US gave France money and improved Franco-American relations, which were somewhat stressed at the time (the US wasn't sure what France's ambitions were in the Americas); for Napoleon to sell the Louisiana Territory sent a signal that the French weren't interested in fighting the US for North American land.
because the U.S offered 15 million dollars

Answer 2

Napoleon wanted to sell the Louisiana purchase to gather money for financing his endless war against Great Britain.