Because while I agree that people have rights and that it's wrong to violate them, rights are themselves derived from consequences and preferences (via contractarian bargaining), and also that "rights" refers to what governments ought to protect, not necessarily what individuals should respect (though most of the time, individuals should respect rights). For example, though in normal life, justice requires you* to not murder a shopkeeper and steal his wares, murder would be justified in a more extreme case, such as to push a fat man in front of a trolley, because in that case you're saving more lives, which is more important.

My main disagreement, though, is that deontology (and traditional utilitarianism, and all agent-neutral ethical theories in general) is that it fails to give a sufficient explanation of why we should be moral.

.* By which I mean something like "in order to derive the benefits of possessing the virtue of justice". I'm also a virtue ethicist.