Someone please explain why Mashima continued Fairy Tail under another title. Why is this under a different name when the 100 Years Quest could also be included in the first Fairy Tail as a different arc?
From storyline point of view, the whole Fairy Tail story was about the dragon slayers, the magic dragon Acnologia and the black mage Zeref. It has already been completed in the main story. Zeref was defeated by Natsu, so is Acnologia.
The 100 years quest has pretty much a whole different story that has nothing to do with Zeref and Acnologia. Thus, adding it to the main story as a new arc would be awkward because the bridging between the new arc and the previous arc will be very narrow.
Most likely because he wanted to begin a new manga but planned to return to Fairy Tail after. This probably changed due to the fact that he found someone to continue it for him while simultaneously working on Edens Zero (the new manga).
Despite the title, the series could continue beyond just this quest as in the new manga, new continents, characters, etc., are introduced and the title may have simply been the title he had in mind or simply liked for the sequel.
It’s because 100 Year Quest isn’t just one arc. It’s multiple arcs that are all part of the quest the new series is named after. For example, there’s the Water God Dragon Arc, and now the Wood God Dragon Arc. This one quest is divided into multiple arcs because there are several objectives and enemies they have to fight such as the White Mage and the Dragon Eaters. The first arc involved fighting the Dragon Eaters, and the second involves fighting the minions of the White Mage(though the White Mage was also involved in the first arc despite not being present.