Well in part because it is a team game which makes it impossible to have "a chosen one" when there are 4 or more players.
This is why I think the current class imbalance works in favor of the game. By mixing powerful classes with weaker classes and adding in rolling for abilities you can easily end up with a "chosen one" in the party. This makes the story better with there being a chosen one and a bunch of supporters.
But Luke was the chosen one in this example and Leia and Han were not really big in the same sort of fashion.
I mean they helped out, but Luke blew up the deathstar, Luke defeated the Rankor and largely rescued them all from Jabba, Luke defeated Vadar in combat