The reason it ended and then got this sequel, I believe is simple to get the design of some characters closer to the one in the LN and anime, the other manga used the characters design of the original publication of the Maou-sama, Retry! light novel (that has only 2 volumes) the new publication has 6 volumes at the moment, the anime was also based on the new LN publication.
We see in this chapter, Aku, Luna, Tron, Yu and Tahara having, whilst close, different designs/clothing.