User blog comment:IamJakuhoRaikoben/Chapter 412: Dance Unto Ishgar, Review./@comment-9595671-20141208153136/@comment-9595671-20141208155638

Another logical reason is he still needs to design what Skiadrum looks like. Remember we've only gotten to see what Weisslogia looks like before in a flashback, and even then he was only in one position so he might not be fully designed yet either.

It makes sense to show them as streaks if that's the case, then again that might not make sense by time reasons.