Because of the lower frequency that would result from the much larger mass collision of the supermassive black holes. Ideally we would have much longer travel

Because of the lower frequency that would result from the much larger mass collision of the supermassive black holes. Ideally we would have much longer travel distances for the gravity wave detection experiment. They work by building a giant interferometer with lasers pointed at mirrors that try to estimate how spacetime gets bent by the gravity wave. This should be easier to do with larger distances and less sources of noise in space. LISA is one such project. So hopefully in a couple of decades will know a lot more about supermassive black holes and other huge phenomena after detecting their gravity waves!