There are actually hundreds if not thousands of conventions held every single year in Las Vegas. I have a feeling it isn't because they have spacious areas for their conventions.
I have my money on the fact that it is because it is a great place to hold one. After the convention there is always a bit of time for some Vegas fun like gambling, shows, a tour or more.
I think that holding a convention in Las Vegas is most likely just an excuse to have some Vegas fun.
I know if I were going to a convention in Las Vegas i would totally make sure that I had plenty of time to actually visit Vegas to really have something to tell everyone back home about.