@Heather Basically alot of older weapons from the JP version used a different outdated type of weapon grinding system. In that system the weapons max grind was +10, and there was a chance of failure.
For the NA version, there are no weapons that use that old grinding system, they only brought the weapons that use the new type of grinding. While they did convert a few weapons that use the old system into new system for the NA version, and rebalanced those few to be better than they were originally on JP, they didn't bother with the great majority of them (despite several of them being from previous games)...
Bringing all old type weapons would not only need to convert them into the new system, they also would need to be rebalanced, since the old type weapons are usually weaker than the new ones.
They also would need to add augment factors into every missing weapon, since augment factors didn't originally exist.