I'll explain, it is because a neutral only carries the unbalanced load on a 120 volt circuit, this is a 240 volt circuit and therefore does not have any unbalanced load so it doesn't need a neutral.
Most residential installs here use a centre tapped transformer wired so that the voltage between line 1 and line 2 is 240v and the voltage between any of the lines and ground is 120v. Since a central AC runs on 240v no grounded conductor aka neutral, (not to be confused with the grounding conductor which the ground) is necessary.
There are 240 volt circuits with a neutral, ever see a 4 prong dryer/range outlet? Two of the legs are your ungrounded conductors, ones your neutral, and the other is your Equipment grounding conductor.
EDIT and Niall commented while I was typing.