Gamefreak said it themselves. They wanted to bring dragon types under control, so they created a new type that is at the advantage to dragon. Rumor has it that fairy is weak to poison and steel, types that arnt great offensively, and resisted by fire and psychic, rather poor offensive types, while resisting fighting, a powerful offensive type. Basically, it appears that game freak is leveling the playing field when it comes to types, so that the overpowered types like dragon are put under control while "weaker" types like poison get their chance to shine. This is kind of like how the introduction of dark and steel limited the reign of psychic types while giving fighting types a chance. Retconning is nothing new either. Magneton was once a pure electric type, but now everyone has accepted its steel typeyness. Our favorite Pokemon while still retain there original typings, but gain new ones as well, and with them likely gain new moves and advantages. In the end, like it or not, the fairy type is here to stay, and you'll get accustomed to the existence of fairies in this world of demons, dragons, giant bugs, walking plants and rocks with fists.
