They forgot they were making a sci-fi and it simply became a weak drama set in space. In TNG you have a similar episode to Tuvix in the episode where Data's status as property is put to trial. They actually made compelling and emotionally moving arguments for both sides of the equation and you walk away from the episode a bigger person for having contemplated a new philosophical quandary that we might not have to face in our lifetimes. That's what sci-fi is supposed to do: force us to consider new ideas. You take that away and all you're left with is a shitty fucking space drama.