Because she needed it. It’s about game balance. You can’t expect everyone else to just adapt to D.Va being so OP that she’s good in every comp, on every map, for years. Your post reads like this to me:
But I like being OP and I hate changing so I demand that every other player adapt to D.Va being OP. Waaaaaaah. This is so unfair. Waaaaaaah. If I don’t get my way, I’ll hold my breath.