Why was celerity nerfed?

Honestly I'm still kind of wondering why celerity was nerfed. I'm someone who loved using the Movement Speed quints and it gave me an additional 4.5% total. That is still less than it was before. Why nerf the movement speed if most people go Transcendence anyway? And why not just put the movement speed back to 4.5% and take out the additional AD or AP buff? If runes are supposed to be even more powerful than before, why not just make it solely movement speed at 5% or something like that? Plenty of items in the game that give 5% extra MS that wouldn't be useful on other champs. Statik Shiv or Phantom Dancer give it, but wouldn't be useful on say a Darius or Maokai or Karthus?

I'm obviously just ranting, but why is it still a nerf from what it was back with the old system? NEW001 said that it would have a greater impact than ever before but then the one thing with miniscule impact is nerfed instead? What is this?