Some NZX listed companies end up decamping to the ASX anyway. Perhaps those NZ businesses who may have otherwise listed on the NZX are seeking funding from private equity funds and/or may then go straight to an Australian IPO with listing on the ASX.
NZ has become a low wage country in an economy increasingly owned and controlled by overseas concerns. I presume these overseas concern see NZ as a low-cost stable environment in which they can extract good profits from the work of their NZ staff.
Increasingly, Instead of investing in shares and business, NZ households have invested their assets into residential land, from which they have had good after-tax returns. To the relatively minor extent that households have super/KiwiSaver funds, only a small percentage gets invested through the NZX.
Low wage economy:
https://www.nzherald.co.nz/personal-finance/news/article.cfm?c_id=12&objectid=12172282