Why Can't you use Grips when using H2H?

1) Grips were introduced specifically for jobs who focused on two-handed (i.e. slow weapons) that were not able to take advantage of what I guess could be called the "discovery" of haste benefit. Heavy weapon-swinging jobs were at a huge disadvantage for a long time while the benefit from Dual Weild (which meant you were NIN, or /NIN using C weapons or something) completely overshadowed using your intended weapon. MNK did not suffer from this situation, because MNK has Martial Arts and has always had tons of access of haste.

2) It doesn't make any sense for you to be able to use grips on H2H weapons.

You guys do realize that they might make it where you can equip 2 different sets of weapons for H2H (like Yang on FFIV) and it would make you guys weaker.

There is a reason why they don't use the sub slot for MNK. Think of having to get 2 Verethragna Lv. 90s.