>>31857802Because it needed a boost to be viable.
I mean, Phanpy is literally just an elephant. Why is it ground type and not normal type? Because ground type works well and makes it stronger, that's why.
Before, Mr. Mime was just a shittier Alakazam. With the addition of Fairy type, he became a kinda shitty Gardevoir. With the addition of his new hidden ability, he's actually pretty damn good and can carve out his own small niche.