Because public polling regularly shows legislators do what donors want, not voters.
New Zealand has political campaign budgets, and limits on airtime. However restricting private citizens is difficult.
There was a significant controversy in the 2005 New Zealand elections regarding budgets. It is alleged US fundamentalists significantly (for NZ lol) funded the Exclusive Brethren Church to produce pamphlets in support of the National Party, by smearing both the Labour Party and the Green Party. https://en.wikipedia.org/wiki/2005_New_Zealand_election_fund...