Comparing the two is like comparing a car to a dump truck.
Both have four wheels, and an engine, but one is more "general purpose" (SolidWorks, car). The other is more "specialized" (Catia, dump truck).
I don't have any evidence, but I would suspect that more seats of SolidWorks exist simply because of the price difference.
Many small and mid sized companies produce CAD data, and use SolidWorks. The cost is "low", and the models from it are easily used for manufacturing.
Catia has a higher cost, and is often used by larger companies to design much larger projects. It also does small projects just fine, but that is like commuting to work in your dump truck :)