Since you asked "how were they unaware", the answer is surprisingly simple.
A lot of people simply don't know (and largely don't care) about being technically a dual citizen. Heck, I might theoretically be a Russian citizen (as an immigrant from fUSSR), but absent a practical need to ensure that I'm not, I won't ever bother checking - it literally seems like a waste of time to me, vs more important things like posting on StackExchange :)
This goes double for countries whose dual citizenship (from the linked article, Italy, UK, New Zealand) is almost guaranteed to NOT bring any practical problems, outside of apparently arcane and nobody-is-aware-of MP rules in Australia.
To bring this answer on topic, it also shows that opposition research in Australia seems rather lackluster. In most competitive US races, a fact like this would have likely been ferreted out by opposition research - or candidate's own opposition-research-defense-team - and brought up during the election (cough Ted Cruz cough McCain cough Obama )