Regarding a conversation where it was mentioned that the changeling queen gave a monologue and sang before anypony attempted to stop her. There's a psychological effect, of which I forget the name of it, that pretty much states that the more people there are in an area or if you have people who are used to being around large numbers of people, apathy kicks in.
I've read cases where women have been raped in front of crowds and a child drowned in a swimming pool with onlookers not doing anything to help. What happens is the more people you get in a group, the less responsibility each person feels toward doing anything. People expect others to pick up the slack or in the case of the above mentioned, expect someone else to stop the action being committed. When discovered, I don't know if the discoverer did it, or someone else showed the results to the Olympic committee, but it's the reason why tug-of-war is no longer an Olympic sport. It was proven that the more people you have tugging on a rope, the less effective each individual becomes I.E., they don't pull at the same strength as they would alone.
If people in cities won't stop to stop a rape, what makes you think average ponies are going to attempt to attack the queen of abominations? Also it was all for drama. Villains giving long monologues is a typical villain trope.