Quite right, I have noticed this too as I recently bought a Bronica EC just for mucking around really, and noticed that there were more of the earlier S and E series cameras and lenses around than the SQ. Still seems to be a bit of the 6x4.5 stuff available, although much of this was well used by the wedding folks.
Maybe the SQ series didn't sell in huge numbers or they are so good that no-one will let them go