It would be considerably overpowered, MNKs are in fact used as tanks these days. Exceptions being perhaps any of the new Hardmode fights like AA and the Super Zilart Bros. As is MNK is a wrecking house OP DD, adding any DD buff to it is in fact pushing them further ahead, which is just something the job does not need right now.

But the reason would likely be balance, Retaliation gains TP Because you receive damage still, as a balance, Countering Nullifies damage completely and deals damage to the enemy, but generates no TP.