In short there is one possible reason and one definite reason...
It is "possible" the creators of Magi may require CR to release the episodes on a different schedule than the original broadcast as a part of their contract. Which they have every right to stipulate, after all, they are the creators of the show. Also streaming anime legally definitely takes longer than doing it illegally.
However the end results certainly make up for the sometimes but not always delayed release.
Because when you watch a show on CR you get the official translation, not a fansub, and are actually supporting the creators of the show you love which helps them to keep making more great anime for years to come.