It just isn't as good as 10-2. That was peak game, that one.

Answering for another user, but in less joking (though I don't hate 10-2 nearly as much as most people seem to), 10 began some significant changes from the Final Fantasy "formula". Notably, it removes the World Map and as a concequence, has much more linear-feeling gameplay (even if in reality it's about the same). You'll note those are the primary complaints about FF13, which I believe is the LEAST liked Final Fantasy (though again, I don't hate it nearly as much as most), so those elements being here as well, as the precursor and water-testing game for them, makes sense.

Additionally, while I enjoy Path Of Exile, and so FF10's level-up system seems simple and straightforward by comparison, the complexity of the sphere grid compared to literally any other FF game could also be a turn-off for some.

FF10 also has I think the most byzantine and arbitrarily annoying sidequests in any FF. Dodge a half-billion lightning bolts in a row. Run a race so fast the timer doesn't even hit "1 second".

Personally, I loved blitzball as the game's main minigame. Probably my favorite "main minigame", but given its sheer in-gameplay value, to those not as enamored with it, the feeling of it being near-necessary might be grating.

The main threat being an obese fish is probably also a contributing factor. Sin had presence and menace, but FF games are RPGs at their core, and RPGs aim to tell a story. The story of the villain there is "he beeg. he fish. he evil.". I know there's a lot more to it than that, but the villain being a semi-unthinking monstruous entity reduces the empathy (or at least antipathy) we feel towards the primary antagonist. Again, a common complaint of FF13: The villain isn't "human" enough.

Finally, and the longest shot and hottest take: Tidus isn't a sexual tyrannosaurus. Most FFs, the main is not only highly desired romantically and sexually by at LEAST two characters if not more, the main actually returns those emotions in a somewhat negative way, giving them flaws. Cloud with his dating sims and underwear theft. Squall not so much returning affection, but has three clear harem members of different romantic stereo/archetypes while being a frigid sigma male. Zidane literally grabs Garnet's ass and comments on the cushion quality. Tidus is much more "pure" in his romance of Yuna. He's supposedly a playboy sports star, but he acts like an awkward pre-teen in half the scenes, and that divergence from the formula might also decrease people's enjoyment of the character, as they are less flawed, and people like flawed characters.

I don't dislike 10.

The more I play it again the more I appreciate the story. Issue I have with it a things that the others had that I miss, like a proper over world. The extreme linearity of the game. The combat system makes the game far too easy, at least in 8 you had to abuse multiple systems to break it/make it easy.

The PS2 games are still loads better then the Crystallis series.