Why are there more diamagnetic substances in nature than there are paramagnetic or ferromagnetic substances?
While the comment of @CuriousOne makes sense, one encounters diamagnetism very often and it makes sense to ask why it is so.
The diamagnetic response to the magnetic field can be traced back to the Lenz law, by which a change in magnetic field always induces such a current in a loop that the lens is repulsed from the field. So the diamagnetic response is is ubiquitous for all systems of moving charges, including the electron orbitals in atoms.
Paramagnetism or ferromagnetism requires the presence of intrinsic magnetic moments that can orient parallel to the field, introducing attractive force. If present, their effect is usually (one or many orders of magnitude) stronger than the weak diamagnetic background.