It seems rather odd and disproportionate for plugin developers to have the ability to include a changelog with their updates, where theme developers cannot.
Whenever I am updating a theme that I have based a child theme on, it's always extremely frustrating not knowing whether any of my child files are overwriting newly-added content from the parent that I would like to have populate down to my child.
This just seems like a no-brainer so I would be curious as to the logic behind the initial decision and also whether this is something that could be changed in the future.
Obviously, if changed, this would require minor tweaks to the core (specifically the updates page, I believe) in order to make the changelog visible in the same way that it is when a plugin update is available. This should be trivial since the change would make the two types of updates almost identical allowing for merging of the core code in some places.