if you read articles at the time of the change, Pokemon heads would follow the progress of the games in many ways, one was by following the official competitions. basically they realized that dragons were too strong based on their use. Ice was too weak, dragon was, well, dragon, and water types that used ice moves did not get STAB (the 1.5 attack/ special attack power multiplier, called the Same Type Attack Boost) a new type was needed. Fairy is in reality a great equalizer (and before you call me a noob or a traitor, I raised a dragonite and a tyranitar so it hurt me as much as everyone else)
Gave dark types a weakness that was specially focused rather then physically focused
Gave Poison types one more advantage
Helped make up for the nerf to steel types
and gave personality to Pokemon that otherwise would be left as normal.
if you want to know why they added a type instead of another fix click here
Hope I helped.