The low sales are hardly surprising. When the 5c was announced, I wondered why anyone other than color-conscious teen girls would buy it. It's too little for too much, particularly in comparison to the powerful and handsome 5s.
If Apple sold the 5c for $350-400, unlocked with no contract required, it'd go like hotcakes to those on budgets, particularly students who later move up to Apple's top model.
As is, it's going to be an embarrassingly failure for Apple.