There's a couple of reasons for this.

- The French conquered the territory from the Austrians, they didn't have any sort of special claim to it, certainly not anymore than they did for their Italian conquests. The settlement after the war generally tried to bring France back to it's borders prior to the revolution, so why would they be allowed to keep the Southern Netherlands* in the first place?

- One of the big principles of the congress of Vienna was to surround France with buffer states, so that if France ever decided to expand again, it would first have to go through these buffers, instead of being able to immediately attack the other great powers. One of these buffer states was the United Kingdom of the Netherlands, which combined the former Dutch republic and the former Austrian Netherlands. The Austrians had had trouble with the area pretty much since they'd obtained it, while the Dutch, and especially their new king William I, were very interested in using the re-ordering of the borders after the war to reunite with the south. At the same time, the British had long considered the southern Netherlands an area of influence of theirs on the continent that they wanted to see safeguarded. They were also of the opinion that the Dutch should play a role in guarding France's northern frontier. Combine this British concern for the region with Dutch proposals sent their way for a reunion of the two parts of the Netherlands and Austrian disinterest in the region, and in 1814, with the 8 articles of London, the great powers created the United Kingdom of the Netherlands. When Napoleon returned briefly to power that year, king William I had already managed to win the elite of the south for his cause, and they insisted on using the victory at Waterloo to gain back even more territory that would have been left to France previously.

-Antwerp. I've already mentioned that the British saw the southern Netherlands as an area of influence for them, and that was in no small part due to the harbor of Antwerp. Not only was it of great commercial value, it was also an ideal place to create and launch fleets from. Napoleon himself had wanted to turn the Schelde river into a "loaded gun, aimed at the heart of England." It was, therefore, unthinkable to leave the area in French hands.

So why was France not allowed to keep the Southern Netherlands? They had no strong claim to the area to begin with, the Great Powers and particularly Britain did not want to give the area to them for various reasons and the Dutch and eventually the people in the Southern Netherlands themselves were pushing for the creation of a new state in the area comprising of both the Northern and Southern Netherlands.

*before the split between the United Kingdom of the Netherlands (the current Netherlands) and Belgium in 1830, the two areas are respectively referred to as the Northern and Southern Netherlands.