I think it's because people's expectations are very high when it comes to main characters, and those are never met.

And then there's the diversity of which people argue about (main) FF characters on these forums; it's not very clarifying. I for one can't stand Lightning, but think Tidus is alright. Just to point out, the main characters are likely to be the most subjective interpretations of any FF game.

EDIT: SpoilerAnd people complain more than they praise, hence the Negativitron-vibe.