I would say because radiation doesn't involve physical contact between matter and so can occur through vacuum.
In conduction you have one or more solid objects in contact. The rate of transmission of energy through the various objects will differ according to the type and thickness of the various objects.
In convection you have normally a fluid in which the transmission of energy is due to the difference in density induced by the change in temperature.
In radiation you have vacuum! Energy is transmitted as electromagnetic radiation (light) and doesn't need any material support. Photons of light of energy proportional to the frequency transport energy from the source through waves that propagates as perturbation of the electro-magnetic field. Once these photons interact with, say, a surface they communicate their energy to the surface.
hope it helps!