Since 6.5 Grendel was targeting both the hunting crowd first, followed by the target crowd, 6.5mm was the right choice for bore diameter/projectile availability. The marketing concept focusing on hunting first was designed to establish more widespread traction since hunting rifle purchases were the main factor in people buying rifles still in the early 2000s, and are probably still that way today.
If it would have been limited to a High Power X-ring scorer and varmint cartridge, 6mm would have made more sense, but would have not realized the same marketability for those wanting to shoot larger game. At the time, it was already a stretch for a lot of Americans to even consider using anything smaller than .308 Winchester, since most were ignorant of the century of success of the 6.5x55 Mauser and early 1900s success of the 6.5x54 Mannlicher-Shoenauer.
Bill A. was also looking at doing a .257 Grendel, but there was hardly any projectile availability for the quarter bore for any target work, whereas 6.5mm had a boat load of exiting projectiles.
