Mostly that we have not had time to implement it properly. There was a patch that automatically applied the upgrade after restoring a backup of a 2.2 assignment - but there are issues with mapping of ids when you do that. Moodlerooms have gone even further and left a re-direct in place so any links to the old module redirect to the upgraded version.
The final solution will be adding code to the backup/restore of the new module so it can natively restore a backup from either module (and then removing assignment 2.2 from core completely). Hopefully this will be done before 2.6.