Everyone: “OMG Naruto rip-off!!! I can’t believe they can get away with this plagiarism!! Stop copying Naruto!!!!!!!”
Everyone: “This is fine”
A few reasons:
1. Fairy Tail is, by and large, incredibly repetitive and generic storytelling that really relies on nothing new, doesn’t really go anywhere, and has characters that learn the same lessons over… and over… and over… all while winning in impossible situations due to “Nakama Power”. (Especially Erza)
2. O-Parts was actually a pretty interesting story that, while starting in familiar territory, took a very different path, and had characters develop in a very realistic way; also, even when characters worked together for a common goal, it in no way meant they were now in complete agreement or “close buds”, as seen in the still-tense relationship Jio and Cross have later on. Something lacking in the Shonen genre.
3. Because Naruto fans are very quick to leap on any and everything they think “stole” from Naruto but ignore any and everything that Naruto took from anything else, calling it an “homage”, or arguing that it was out first so couldn’t have taken from a manga released after it.
4. Because Naruto, by comparison, is also quite generic and has fallen apart heavily in the second part, especially the final arc, and yet O-Parts is accused of having the “horrible and rushed ending” despite what was foreshadowed throughout finally coming to pass; this is primarily because it’s not the stereotypical “Shonen Characters Always Find A Way!” narrative that people claim to hate and yet get mad if it isn’t present for the sake of characters they like.
5. The ambiguous ending is also often criticized because it does not spell out a 100% definitive ending or fate for certain characters/makes it seem there was room for more that wasn’t done. Should be noted Dragon Ball ended the same way and yet managed to become one of the most beloved anime/manga of all time, and the writer, Akira Toriyama, said he ended it in such a way that the story could live on in the imaginations and hearts of the readers, that it was only as limited as they wanted it to be. It should ALSO be pointed out Seishi said many times that was one of his favorite series growing up, and Toriyama was a HUGE influence on him and his work.
6. It deserves an anime but likely won’t get it for the BS reason that people will accuse it of “ripping off Naruto” there as well. Despite Fairy Tail not receiving the same stigma. However FT has one thing in it’s court: an over-abundance of “sexy women” and a crapton of fanservice, even in the artist’s sketch work. Fanservice sells anymore, hence why every other anime out anymore in a generic fanservice harem series. It’s sad, but… there it is. I’m not saying that’s all FT is, but it certainly helps it.
So yeah, there’s a number of possible reasons. lol