See any freshman physics book, or one more advanced, to learn that electromagnetic waves are transverse, and that does it --transverse => two polarization states. Very basic stuff.
It's called "helicity operator eigenvalues". Why the photon has two instead of 3, well, it's called "gauge invariance". "Photon polarization" is a bit of an oxymoron, as "polarization" is typical to wavelike phenomena, while "photon" is a particle.