LIGO and VIRGO are sensitive to gravitational waves within a specific frequency range. The bulk of GW energy is emitted at a frequency that is twice the orbital frequency of the objects emitting the gravitational waves. Supermassive black holes are sufficiently large that they merge before they could have an orbit compact enough to emit gravitational waves at high enough frequencies for LIGO and VIRGO to detect them. Less massive compact objects are smaller and so can reach higher orbital frequencies before merging.