Using Daggers with Flurry of Blows.

I am thinking of a Monk that gets Weapon Mastery Dagger at level 4 SO he can make an extra attack with is attack action (3 at level 5).

Since daggers do the same damage as unarmed attacks. And nick only works during the attack action.

Why can't a monk use the dagger with Flurry of Blows. I do not want to wait for a Kensi Update that has this feature.

Anyone know why this is a rule?

Anyone allows this in their game ?