I imagine it was the legal issues of standardizing the 6mm Grendel. In order to not have IP battles with a variety of entities or the liability of existing systems out there that would be interchangeable with the "new" round but not standardized, it made the most sense to change it a small amount and move forward with a unique set of specifications that would not directly conflict with anything currently available.It's more a matter of having to have this discussion at all. If the cartridges are so similar that you even have to think about this, then you can probably just use the older cartridge. I'm all for innovation though. If it catches on, great. the mid-sized 6mm market is getting REALLY crowded though...
I agree that the small 6mm space is pretty well saturated for reloaders and wildcatters, but not for the off-the-shelf general users. I'm happy that things are moving this direction instead of towards larger and larger magnums for hunting use-cases that I and many others have no use for. I could see an exciting new world of factory-class competition coming out of something like this - it's the closest thing to a factory 6 PPC and is quite likely going to get A LOT of support from the mainstream gun industry. This could be a great way to bring new people in to BR type competition. What if GMM makes 6ARC ammo that, in high quality rifles, aggs in the 2's? Like an Berger 68FB off the shelf for short-range. Or a 95 or 105 for mid-range. What if a factory IBS class existed where that is competitive and the focus is on flags and gun handling? You could get more people excited about competing if that were the case. This round has a better chance of that then other SAAMI rounds.