User blog comment:Eddienam/Question/@comment-12642484-20131226052602

Because most of the popular manga are released in manga magazines. There are different types of these magazines. OP, Naruto and Bleach are part of the "Weekly Shounen Jump", which, as the name already says, is a weekly magazine.

Attack on Titan is being published in the "Bessatsu Shounen Magazine", which is a monthly magazine, hence the chapters are monthly.

The only way to change that would be if Isayama, the author of Attack on Titan, decided to switch to a weekly magazine, however because of contracts with the Bessatsu magazine, that is most certainly impossible.