Because a character centered around a single personality trait is dull? Sure, he's selfless. So what? Go check out the "nice guy" thread in EoEE to see just what people think about someone who is "nice" and nothing else.

Zidane is boring. He has little depth and less growth. He's there, he's nice, but, frankly, so what?

Tidus was whiny, sure, but he had a reason to be, and he grew out of it. He had an interesting character arc, accepting the loss of everything he knew, coming to terms with his father, accepting the burden that the world put upon him. He was at least a person. Everything about him, including his complaints, felt real and made sense.

Zidane was just positive and happy-go-lucky. And...that's about it. No reason for it, no substance to it. He just was.