I think Tidus is and should be considered the main character in the game but Yuna's character is so well-written and so well-developed that i can really understand it if someone considers her as the main lead.
Tidus is the first character that you get to control and you do so for most of the game.There's a rule in movies (as well as games) that you have to show your main character (protagonist) as soon as possible.The developers also try to make this clear by having him say a couple of times "this is my story" (however egoistic this may sound).
Yuna is one of the best secondary characters i've ever seen in a game...She helps the hero to discover and develop himself as well as to push the story beyond.She is the main reason that drives Tidus even more than his confrontation with his father.
I believe that those who like Tidus as a character they do so because of Yuna.She really helps to make him look good.