Cause it doesn't fit their investment strategy. Which is, to my understanding, to invest in stable companies that provide regular income for them to pay for injuries.
https://www.acc.co.nz/about-us/our-investments/
We favour long-term investments that can deliver relatively certain income streams over long periods of time. These investments match our long-term cash flow requirements, and also help offset the risk of declines in interest rates.
They'll be dumping more soon. I'll have my pan ready.