Bartz definitely gets the least character development. Maybe that is why he is considered the "leader"? Like Chrono, or Cloud, the mostly silent protagonist that you role-play as. At least, I don't remember Bartz saying much in FFV.

But I actually don't think there is a leader in this game or in many of the other FF games. FFVI, it was argued on whether Terra, Locke, or Celes was the leader. In a way, Dissidia has established Terra as the leader in FF canon, but I don't think that was the original intent. Obviously, those three characters are the main characters of the game though. FF didn't return to this formula until Final Fantasy 12, and I think the only reason photos of Vaan were prominent when advertising the game was just marketing. They wanted to make an illusion of a main character, following the popularity of Cloud, Squall, Tidus, etc. But Vaan isn't the main character, if anyone is, it is Ashe. Finally, Lightning isn't the "leader" of Final Fantasy XIII either. She is on the box and pictured the most. But she doesn't have a greater bearing on the story than any of the other characters. She is "marketed" as the leader... in fact, female cloud. But she's not.