Probably because the initial conception for 14's classes was to keep as far away from traditional FF jobs as possible, hence the introduction of rarely used FF jobs like Gladiator or completely new jobs like Conjurer and Thaumatarge.
When they later redeveloped the classes the only dagger wielding class (GLA), didn't exactly get stuck with Thief like abilities, and frankly most people thought it would turn out as a secondary job title to Pugilist. However there IS a few dagger types that aren't being used such as Kukuri, Edge, and perhaps Blade weapons which a Bandit/Thief Archtype could utilize.
Either way they're reworking Monk in 2.0, so it might incline they're re-working it so there will be some distinction in abilities between THF and MNK if they choose to bring it into the game in the future.
