I suppose the .750 barrel became “standard” when the Marine Corps adopted the Government profile barrel.
The problem is that there aren’t many .625 gas blocks or front sights out there. It would be easy to have barrels turned down to the proper .625 diameter but the other hardware just isn’t readily available.
And that’s a shame because the AR-15 is supposed to be a light rifle. I’ll grant that there are special roles (varmint, target, sustained rapid fire, etc.) where the benefits of a heavy barrel outweigh the drawbacks. But for general use the light barrel is better.
Besides, having a 5.56 rifle that weighs as much as a comparable 7.62 rifle has never made a lot of sense to me. But, maybe it’s just me.
Anyway, it would be nice if someone came out with a full range of .625 barrels and accessories. Just an idea for some small business looking for a niche in the AR marketplace.