The problem with having something other than Ninja's gimmicky Sneak Attack that deals more damage in the front is confusion. It confuses people who might think they are suppose to attack from the front, only to be cleaved for their trouble.

Giving Monk more damage while they're at the non-corresponding position, is a slippery slop of leeway which might result in a situation of not using correct positioning at all, because the damage is only marginally different.