Grips were introduced to try and counter balance the 2 handed update. SE nerfed two handed weapons due to people complaining they were too strong. Then SE realized they nerfed it too much and so introduced grips to correct the mistake rather than just reversing the update. However, the players who benefitted the most from the addition of grips were mages.

Seeing as how mnk weapons are not considered two handed weapons, they were excluded from the two handed nerfing, and so did not have any reason for a counter balance through grips.

That is the history of the 2handed nerf, and grip introduction as I remember it from 2006-2008.