That's not an implementation choice, that's forced by the fact that you used an external link.
You have a parent document A, which references another file on disk B. You modify B and close it -- Photoshop has no record of the previous state of file B. Then you undo the update of file B in parent file A, which undoes the visible changes in A, but cannot change file B.
If the child document were embedded, then Photoshop would have a record of the previous state of the file, and be able to revert back to that previous state.
But because you chose to use an external link, Photoshop cannot revert that external file, because it has no idea what the previous state of the file was.