Nations didn't just want land for land's sake, they wanted the resources attached to that land. The primary resource produced by Louisiana in the 18th century was grain. Louisiana was a huge stretch of land, and it takes money to administer and defend such a large territory. And grain's not a particularly valuable or rare resource. So the real question is what use did France have for Louisiana in the first place? The answer is that Louisiana wasn't anything special on its own, but it was incredibly valuable for supporting France's more important colonies in the Caribbean, particularly Saint-Domingue (modern Haiti).

Saint-Domingue was a tiny colony with a dense population, most of whom were slaves. There wasn't much farmable land there-- most of it is mountains-- but what farmable land Saint-Domingue had was some of the best in the world. In particular, it was perfect for growing three valuable cash crops: coffee, indigo, and most importantly sugar. Saint-Domingue produced about 40% of the world's sugar at a time when sugar was an expensive luxury. It was the most valuable colony in the world, and France wasn't going to waste one inch of that scarce land growing cheap crops like grain. So instead France used grain from Louisiana, which wasn't as good for stuff like sugar, to feed their Caribbean colonies.

But in 1791 the slaves of Saint-Domingue revolted in what would become the Haitian Revolution. Ex-slave armies destroyed many plantations and drove most of the landowners off the island, and Spain and Britain leapt at the chance to try to conquer Saint-Domingue. By 1800 the island was back under control, under the leadership of the former slave general Toussaint Louverture. Toussaint was nominally loyal to France, but maintained a great degree of independence, and Napoleon sent his brother-in-law to reconquer the island and re-enslave the population. When this expedition was crushed by a combination of yellow fever and savvy guerrilla warfare, Napoleon gave up on Saint-Domingue (which in 1804 became the independent Empire of Haiti).

With no Saint-Domingue left to feed, Louisiana was a lot less valuable. Napoleon might have kept it around, except that he fully expected that either Britain or the US would eventually conquer it anyway, and France was stretched too thin by the Napoleonic Wars to defend it properly. So rather than wasting time defending it only to see it lost for no gain, Napoleon decided to throw the US a bone and sell it to them on the cheap.