Predecessor is not for the tasks, but for the PBO. You might be able to get what you want by displaying the users for the various roles that the workflow uses. It wouldn't give you exactly the one user immediately previous to the current. That is probably only visible in the object history or through the workflow itself.
If you want to get exactly what you want, you would need to develop some java that finds all of the related tasks and then selects the owner of the most recently completed one. Wouldn't know where to direct you from there though.