Why are territory voters only counted in the national majority in a referendum?

That is an interesting question, Abigail. 

Section 128 of the Constitution says a referendum is passed if it is approved by a majority of voters across the nation and a majority of voters in a majority of states — a double majority. A double majority makes sure there is support across the country and that voters within every state, regardless of their population, have an equal say on the change. The votes of people in the Australian Capital Territory and Northern Territory are only counted in the national vote.