They did. Then later on nationalised them again.
Upthread comment: >[privatisation is] one of the UK's most hated things the government has ever done
In New Zealand, a lot of the political fallout from privatisation came from older people who love trains and talk a lot of crap about how good they are. They are uneconomic in New Zealand, regardless of how efficient people think they should be.
Voters don't like reality (few have any working knowledge of business), and most politics seems to be about finding alternative acceptable ways to present reality to voters.
And why is that?
I think it's probably also correct to say that New Zealand could sustain a larger passenger rail network if made the right investments. For example, Auckland <-> Hamilton <-> Tauranga could profitably support at least a few trains per day in each direction... _if_ it was electrified, with passing loops where needed, and with well-sited stations, and with the ability for trains to run into Britomart or at least Newmarket (both being stations close to central Auckland). However, there's a chicken and egg problem: unless you make investments like that, the service you can offer will be pretty crap and no one will want to ride your slow trains.