Look at dragons for a second.
![](https://www.pkparaiso.com/imagenes/xy/sprites/animados/reshiram.gif)
When you see a dragon like Reshiram or Dragalage, you think of mythical creatures, right? That's what dragons are: Mythical Creatures. Game Freak most likely wanted to reflect the elusiveness of draco in some of it's more powerful Pokemon, like psuedo legendaries Dragonite, Hydreigon, and Garchomp.