The basic problem is that there are several ways you can handle someone being disarmed, and they're all bad.
1. Its hard to do, but it ends the fight. He has no weapon! You do! That means he loses now. What's he going to do, bite you? Punch you? You've got a yard of steel. He's screwed. The problem with this approach is that it completely bypasses hit points. Its essentially "save or die" in a new form.
2. Its not that difficult to do, and the disarmed opponent suffers a small penalty, either by drawing a different weapon, or artfully recovering the original weapon. This has a couple of problems. First, disarming stops being a meaningful penalty if everyone has multitudes of replacement weapons, or if everyone can just regain their original weapon without meaningful inconvenience. It may even be completely pointless if a replacement weapon is just as good as the original. Second, it makes fights seem like a farce, where everyone is continuously dropping their weapon but no one can manage to capitalize on it.
Obviously the other two possible combinations (its easy to disarm, and ends the fight, and its hard to disarm, and causes only a minor penalty) are even worse.
To design rules for disarming, EVEN BEFORE you consider how to make those rules mesh with monster design, you need to figure out what the penalty should be for being disarmed, and then to decide whether that penalty is reasonable in the overall context of the game, and how difficult it should be to inflict.
