I couldn't find it in the forums and I wasn't sure if this was just me. it's been there for a while, and I was wondering when/if it would get fixed :/
The reason why this is disabled is because there is an issue with items stacking incorrectly. We haven't been able to fix the stacking issue in this source, and felt it was safter to disable inventory sort, as using it caused items to restack correctly, often with the unfortunate result of items that had overstacked being lost from your inventory. We felt on the whole that it was better that players didn't lose items, as there's no record of which items are lost from your inventory as a result of using inventory sort while having wrongly stacked items in there.
Ah, I guess that means I have to actually sort my things now. I've been putting it off, hoping it'd get fixed eventually xD thanks for letting me know!