There is no overriding reason why magnetic charges (monopoles) do not seem to exist. It is straightforward to supplement the known laws of classical electrodynamics with magnetic as well as electric point sources. Moreover, in quantum electrodynamics, the existence of magnetic monopoles would actually solve a known problem, since the presence of magnetic charges would require electric charges to be quantized. As a result, physicists have spent a significant amount of effort searching for isolated magnetic sources.
However, they have never been found. It seems that our universe may simply have electric charges but no magnetic charges, for no deep underlying reason that we know of. The most we can say is that if there are magnetic monopoles that we have not yet seen, the reason is probably that they are very heavy. In many extended theories of particle interactions, there are monopoles, but the monopoles have masses much larger than the masses of the electrically charged particles. They are therefore difficult to produce, except in incredibly energetic interactions or in the early universe, when the temperature was very high.