The pool had safety issues and was closed down in 1985. Apparently there is a vault under the pool and officials feared a collapsed. Hence, the mayor of Roubaix rezoned the structure as a museum.
A better explanation, summation and a slew of photos can be found at kuriositas:
So, the repair work was undertaken and ten years ago in October a vast crowd attended the re-opening of La Piscine as it is called. The first year exceeded all expectations. Around 80,000 visitors were anticipated but the museum (the La Piscine-Musée d'Art et d'Industrie André Diligent to give the place its proper name) drew over two hundred thousand.