As soon as charge is quantized, this will happen. In any quantization scheme you will have some smallest charge. There are particles with charge +2 (the Delta++, for example), but ... anything that can decay while preserving quantum numbers will decay, so you end up with protons in the end. (ok, the quarks have fractional charge but that's not really relevant at scales we care about QED)

If the question is, why is quantum mechanics the correct theory? Well, I guess that's how our universe works...