User blog comment:IamJakuhoRaikoben/Chapter 415: And It Shall Become Your Will To Live, Review./@comment-4449950-20150110235458/@comment-76.161.172.226-20150112213722

Jellal's purpose this arc was to recruit Oracion Seis, and serve as the reason Tartaros didn't simply kill Erza, or stick her unconscious in a tank and start experimenting on her like they were going to do with Mirajane.

The end of the GMG arc set up Cobra using knowledge of Tartarus as a bargaining chip to free the Oracion Seis. The beginning of this arc was time for that to take place, and also time to show the aftermath of setting them free. Which is to say, Cobra killing the treacherous Brain, and Jellal showing up to let his former victims beat him up for a while before beating them and giving them his recruitment pitch.

Anything further moves beyond the scope that needed to be covered unless they were to be directly involved in this arc. Given that they were starting - injured - in an entirely different country not long before Tartarus attacked Fairy Tail, with no way to know what was transpiring, there's no surprise they weren't further involved.