I thought stunning fist was a Monk ability ... monks are WIS based?!
So the way that Sacred Fist works is that it's a paladin base that gains Monk class features at specified levels (AC, speed, flurry, etc) and anything based on WIS from that list is based on CHA instead, and if you multiclass you can get either or.
Previously the Epic level martial arts feats that required flurry of blows were locked to Monk only at level 12, but they added Sacred Fist finally earlier this year. Since SF is supposed to essentially be an unarmed paladin using monk features as a unique class, having Stunning Fist not work off CHA but WIS only puts a small hamper on the standard unarmed toolkit available to monk. There's a reason why alternate ki weapon feats are more popular on monk builds than unarmed, so keeping the primary boost for unarmed in helpless damage from Stunning Fist on WIS limits a lot of the strength that SF could have on its own in higher tiers of content.
Right now, SF is relegated to a very strong heroic class that falls off in Epic and Legendary content. It's not a unique issue with SF, but it becomes more noticeable as the baseline for Stunning Fist becomes harder and harder to achieve without more investment than it should.