Why is a Capital Gains Tax such a political hot potato?

Labour first started working on a blueprint for a capital gains tax under David Lange, but it was scrapped after Labour's 1990 election defeat. Fast forward to 2019 when Jacinda Ardern ruled out implementing the tax while she was prime minister So it begs the questions, why is a capital gains tax such a political hot potato? Is this a good political move for Labour? If the public show support, could it be something National adopt? Dr Bryce Edwards, Bryce is director of the Democracy Project and a political analyst in residence at University of Wellington.