Dragon-types were the strongest offensive type in Gen V. You can spam Outrage and Draco Meteor because only Steel-types resisted it, and even then those Steel-types are weak to Earthquake and Fire-type moves that Dragons often carry. Game Freak created the Fairy-type to weaken Dragons, as well as Fighting- and Dark-types, since all of them were pretty powerful. The immunity is added as an bonus so that Dragons can't spam Outrage or DM anymore.
For the same reason, Fairies were weak to Poison and Steel because those types were offensively outclassed prior to Gen VI. Poison was only super-effective against Grass, but so were 4 other types - Ice, Fire, Flying, and Bug - and each of those types was weak to at least another type, making Poison outclassed. And Steel was super-effective against Rock and Ice, but then again so is Fighting, which is also super-effective against Dark, Normal, and Steel. Steel and Poison now have an offensive niche that ensures no other type can outclass them.