I think they would tell you that most ammo being shot isn't the 'good stuff'. Most of it is cheap 55gn FMJ and 62gn surplus, the ammo where I personally find more fliers with my 1/7, and I presume, so do the manufacturers. I can speak for Stag Arms take on this, because I asked the CEO exactly this question, and his response was "Because I don't want to have to spend my days on the phone explaining to shop owners why their customers rifles won't group well with the most common ammunition they sell." This is also when I was told that their 1/9 is closer to 1/8.5 and should stabilize most any mag loadable ammo.
The vast vast vast majority of 223 ammo commercially sold in the USA is <62gns, and the 1/9 is superior there, that's why almost everyone offers the bulk of their lineup in 1/9.
As Leg said, the US Military went with 1/7 due to the length of the tracer round, not due to better accuracy with 62gn M855 ball ammo. I know we have a few customers who put 20" 1/9 uppers on their C7 lowers for matches, as they find they shoot C77 ball more consistently.
