You know, sometimes viewing figures and popularity of a show don't really matter. If a commissioner really hates a show it's doomed however well it's doing. Or so I hear anyway. Obviously I'm not saying that's definitely the case with NGO.
Or sometimes it's down to a shift in power, where a new commissioner replaces an old one and wants to shake things up, and by that he or she has to get rid of existing slots to make way for his / her own.
It's all politics and beyond the comprehension of most of us mere mortals.