Storage limitations from the 80's (as described by sibling comments) is one reason, but there's also another: which characters are considered their own graph (v

Storage limitations from the 80's (as described by sibling comments) is one reason, but there's also another: which characters are considered their own graph (versus variants) is a somewhat fraught geopolitical issue, especially when China and Taiwan are on opposite sides of the argument. The final chapter of Yale professor Jing Tsu's Kingdom of Characters does a great job laying out how this manifests itself at meetings of Unicode working groups.