Supermassive black holes are larger, so the characteristic timescale of their dynamics is longer, and the gravitational waves generated by their merger are correspondingly of lower frequency. More specifically, supermassive black holes binary (SMBHB) mergers usually ring at 0.1-1 millihertz, whereas LIGO's sensitivity is concentrated around more like 0.1-1 kilohertz, a typical frequency of stellar-mass black holes binary mergers. The ~6 orders of magnitude difference comes directly from the fact that supermassive black holes typically weigh in at millions of stellar masses.

See the plot here:

http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.ht...

(To convert from wavelength (meters) to frequency (hertz), take the speed of light and divide it by the wavelength.)