Comment by exmadscientist

This is called "charge quantization", and it is not definitively explained by modern theories. There are some very good arguments for it, to be sure, but I don't think they're quite case-closed, of-course-it-must-be-that-way good. It is related to C symmetry, as a discrete symmetry, which ties in to Lorenz invariance and all that, so there's that angle too.