Security in a way. The issue is that there’s no way for the properties UI to display those types of items even if the Attribute storage itself can store them. It’s not just a question of implementation work either, we couldn’t arrive at a UX we were happy with for dealing with them so excluded them from the first version of Attributes rather than making people wait even longer for them.
TL;DR: Having people able to pack hidden data into an instance that you can’t find out is there without scripting is not ideal. We did it with Tags, but didn’t want to repeat the same mistake with Attributes.