The OU where these User objects reside does not have a User Deprovisioning Policy linked to it, or the existing linkage is Blocked.
To check for linkages to an existing Deprovisioning Policy in the MMC, select View | Advanced Details Pane.
Browse for the OU of interest and check the ActiveRoles Policy tab at the bottom of the Advanced Details Pane. Deprovisoning Policies have a red arrow on the icon and should be easy to spot. Make sure that there is a User Deprovisoining policy and that it is not currently Blocked.