First, I don’t think science can really answer questions of the form, Why didn’t _____ happen?
Maybe, but it certainly should be able to explain what it's trying to explain - in this case why the dinosaurs died out. The impact isn't an answer to that question because it doesn't explain why other groups didn't die out and thus leaves the question unanswered.
Second , I think there are very good hypotheses about how each of those types of organisms survived: omnivory, small body size, low metabolism, and/or ability to find shelter are all considered likely explanations for why various groups survived.
It is simply not possible to find clear dividing lines like that between groups which survived and groups which didn't. While the popular face of dinosaurs might be the big beasts, there was a whole diversity of different dinosaurs out there, from tiny omnivores to massive sauropods. A fact that becomes increasingly pertinent as it becomes increasingly clear how similar the smaller theropods were to birds.