User blog comment:IamJakuhoRaikoben/Chapter 415: And It Shall Become Your Will To Live, Review./@comment-24102956-20150112215027/@comment-7418318-20150115091938

I have a few theories on that matter, all of them only spaculation on my part.


 * His strongest Demons are so powerful that their power is eroding the bodies over time. Unless they are returned to their book-(or some other inanimate)-shape, they'll fall apart sooner or later. The Demon Gates were specifically built for long-lasting operations and thus Zeref limited their power to a degree where they could function for centuries without needing to return to their book-shape.
 * Stronger Demons are much harder to revive. The Tartarus members were created in a way that allowed them to easily be revived in "Hell's Core" so their battle-experience would be lost. While they lack power compared to the stronger Demons. Zeref hoped that their lack of power would be compensated by the vast amounts of battle-field experience they could ammass in this fashion.
 * The Tartarus Demons were never meant to destroy Zeref. Their real purpose was to take care of END while it was vulnerable, then revive it and provide support for it.
 * When Zeref created the Etherious, he learned a few new tricks that enabled him to create Demons even stronger than them, which led to a new generation of Demons that are like END and thus much stronger than the Demon Gates.
 * The stronger a Demon is, the longer it takes to create it. When he created the Demon Gates he needed (or wanted) troopers quickly and thus settled for low to mid level Demons. Now he had a period of more than seven years in which he could work undisturbed, allowing him to finish the very time-consuming process of creating high-level Demons like END.
 * In the past Zeref created much stonger Demons, but back then he only created them as easy to control foot-soldiers and thus didn't give them the intelligence to operate on their own without orders. Due to their lack of mental capacities, those that were roaming free didn't survive the 400 years, but he turned some of them into books and stored them in case he ever needed them.
 * Zeref knew that after they had fulfilled their purpose of killing him, the Demons would run wild and kill many innocent people. He made them just as strong as he thought they need to be to fulfill their purpose, but not any stronger, so the damage they could wreak after he was gone would be limited to a minimum and so the Humans would be more likely to be able to destroy them.