Because the path finding for Charge basically walks you super fast along the shortest uninterrupted path. If you aren't on the ground the path finding algorithm can't find a solution. Same for when there are obstacles in the way, e.g. when standing on some objects that are connected only crudely to the surrounding terrain. This also means that if there is some roughness in the terrain your character may choose to charge the long way around, which looks goofy and takes longer than the stun.