Why won't Labour keep its promises?

Back in 2017, Jacinda Ardern promised "There will be no new mines on conservation land". NZ First stopped her from doing it in her first term, but since winning an absolute majority in the 2020 election Labour has refused to move on it - and has explicitly voted down a bill which would implement that promise. So why? Its not as if keeping conservation land for conservation is unpopular. Quite the opposite:

But a new poll commissioned by Forest and Bird found the majority of Kiwis (66 percent) think the Government should implement its 2017 promise to stop new mines on public conservation land. Sixteen of the respondents disagreed and eighteen percent were undecided.

That's a pretty big public mandate, and more people than voted for Labour and the Greens in 2020. Actually keeping their promise would be a popular move. So why won't Labour do it? Do they work for their voters, or just for a small clique of big donors?

This sort of betrayal undermines trust in the Labour Party, and in the political system as a whole. It builds the impression that politicians are just liars. But its also a driver behind Labour's fall in the polls. After all, Labour clearly doesn't care about its voters. So why should they care whether Labour stays in government?