Oh come on now! You ask why other leaders can't step out and try to become PM? Surely you know the answer to that. The very honest people won't go through the required bribery that it takes to build up the numbers, and they wouldn't have access to the kind of blood or sin money that people like Namah, O'Neill, Somare, Chan, etc have always had access to, in order to bribe.
They've all sold their souls to the devil but they don't care. They have no ethics, no shame, no sense of hypocrisy. It has all disappeared from them.