Perhaps because all successful managers throughout history have possessed certain personality traits that are needed to manage at the highest level, which have been evident to see in conferences and interviews that they do. Unless you have zero life experience, you should know that to be a good manager you need a certain level of charisma and to be have an authoritative presence. It can't be learned in a text book or on a course, you either have it or you don't. Giggs lacks any of it. You don't have to be best friends with the man to see that.
I don't know why some people get so defensive about it. No one is insulting him, we're just commenting on obvious shortcomings that are relevant to being a manager. It's why likes of Steve Bruce and David Moyes will never be particularly good managers but the likes of Mourinho and Guardiola are. There's just something special about them that makes players want to listen and follow them. They instil confidence and a belief that anything is possible. And then there's Ferguson, a man who when he talks everyone around him will stop and listen; he commands respect. When Giggs talks I want to mute my TV.