KAC was dug in tight with the unit since the 90s. Its nice when a big part of your R&D is the best funded special operations unit in the world, with their own huge R&D budget and operations. We can in large part thank KAC and Delta for the reinvention of the large frame AR in the 90s and the development, advancement and standardization through the 00s and 10's. I think with the K2 generation of guns, we kind of hit peak performance and it wasn't until some radical changes (such as phased gas array and a barrel that can shoot that tight for that long) that we see improvement with the large frame.
The other thing is the advancements in the small frame with calibers like the 6 and 22 arc, are really taking away the advantage of a large frame AR, unless you need the energy downrange. They are much smaller, lighter, standardized, easier to shoot and have way more options for build out. There are going to need to be serious advancements with the large frame to make it worth it. But the funny thing is all the advancements that make the MRGG so great, are being rolled into the GFR small frame.
If It was my money, today, I would buy a 6 ARC GFR or wait for the 22arc that's currently being tested to roll out.