Why are Fairy Tail and Fairy Tail: 100 Years Quest separate series? 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.