Due to some of the following reasons:
- Other than Steel, Dragon is resisted by absolutely nothing, and Game Freak just entered the most appropriate resistance for it. Basically they needed a filler, or Dragon types would prove too overpowered.
- Logically, a dragon's claw would leave a scratch in steel, which is not nearly as effective as Fighting moves that can break it, for example. So the moves that Dragons use are generally things that strong Steel could take a hit from.
We don't know for sure since Game Freak are never going to reveal this type of thing to us, though in theory there are the reasons that Steel resists Dragon.
Remember, Game Freak is naive, and they believe they can add what they want to the game, but with no logical reason. -.-