Ann: S&P DJI Announces March 2019 Quarterly Rebalance, page-4

Funds that hold baskets of stocks within indicies to track those indicies like funds tracking the ASX 300 can no longer hold it.

Also advisors & trusts usually have rules about where fund money can be invested. Because it's outside the 300 it potentially wont meet some of those requirements.

Perhaps other scenarios too (like overseas brokerages only offering access to the top 300 ASX stocks - this one is just a guess)