I counted how many Dragon, Darkness, and Steel types that are out right now and I came up with a total of 29 Dragon, 36 Dark, and 35 Steel (That's including Genesect). 29 for one type isn't that bad, when Dark and Steel were first introduced we only had Murkrow, Sneasel, Steelix, and Scizor, which explains why they made only special dark and steel energies. It wasn't until they made a bunch of new steel and dark pokemon and changed some of the older pokemon to those types that we got basic dark and steel. It's also possible that they are thinking of creating a lot of dragon types for Gen 6 but that won't be for a while.