Supaga have another problem when it comes to leadership. Rikataso was leader less than a year ago and is still in the group. The other 1st gen members need their time to shine too so although Koume is the best candidate, it would be terrible for the older members to be overlooked for the position. Making Koume subleader until the next leader graduates would be a good idea too. In the end, the least controversial move they can make is giving a 1st gen member minus Rikataso the leadership.
In retrospect, it was stupid to entrust Amita with the leadership if they knew she was going to graduate in a few months, even if she really deserved it. It must have been a really sudden decision or they were desperate to keep her in the group by giving her the leader position.