Your guess is as good as mine, but the most likely answer is just balancing reasons.
..........that's a fair assumption to make, but I can't see that as a strong enough argument that could be made for this case. In my vanilla experience playing through CoCII, outside of save editing you can't ever really be "overpowered". I can see how strong the perk could potentially make some late game warrior builds by letting them use stats from 2 two-handed/non-light weapons, but even then the enemies we'll be facing by the time we can even use this
9th lvl perk will more than be able to handle said stat combination. Many of the better 2H unique weapons have a negative stat to counter how effective they perform, the stat tank from two weapons could in itself prevent any build from being too strong, it'd either require a attribute or equipment change to compensate preventing any one stat from ever being too too high.
2handed weapons are balanced with the assumption they will occupy both hand slots.
Also, it may or may not be a pain to code.
If need be, I will gladly code it myself I'd be more than happy to. But I'm not just talking about two-handed weapons here. Like
@Shrike675 mentioned, warriors being able to monkey grip would let them use
any weapon in either weapon hand slot, so you do two shields, two quarterstaffs, two non-light one-handers.
Honestly the perk is more for character headcannons and less how effective am I in combat. What with lvl 7 granting 2 mutually exclusive options for perks - for warriors, Tough Skin & Cleaving Strikes offering PCs a defensive or offensive path of play - I figure lvl 9 will more of the same, just another offensive or defensive perk to futher specialize your pc's type of fighting style. So if Giant's strength is the offensive side, the defensive side of that could be a perk that grants warriors passive health regen once drops below 50% and +15 armor, +15 power & +15 evasion so long as the health is below 50%, call it Battle Junkie.