6mm PPC --> 6.5 Grendel --> 6mm ARC - Why?

[Fess]

As Grayfox noted, the 6mm ARC was intended to get a long-range man-stopper out of the AR15 platform. Hornady designed it around their own bullets and wanted to get as much case capacity as possible, so, like the Grendel, they pushed the shoulder of the PPC case foreword.
There were other reasons, of course for making variations of something that already exists. This scene from "Spaceballs" comes to mind:
https://www.youtube.com/watch?v=aN8wxQDqfRk