Always assumed it was just cause mnk lacks the spike war/sam can offer either through warcry/MS, or hybrid exploiting/yaeg for a final push.
Also mnk being just slightly squishier in WS sets since its not simply braindead stack WSD nyame/emp+3 in every slot, and since its not all frontloaded into the 1st hit this leads to variance in WS numbers just due to missing a hit/not getting a good MA roll, etc.
Job aint bad, the other 2 meta-picks are just more consistent and can spike quicker through JAs in a time-strict event