User blog comment:CNBA3/My Balam Alliance Arcs/Dark Knight Trilogy Marathon Complete!/@comment-213.89.153.169-20150208213818/@comment-7418318-20150209150235

If I remember correctly Mashima once said that those flashbacks were something he couldn't add to the manga, though he didn't state a reason.

Maybe the editor was against it or he didn't have any "pages to spare" at the moment.