The same reason that there is only one UN governor general... What's hard to get? It will be a ceremonious posting with no real power. But individual governments would vie to have their own representative elected to the position. This is the same reason that baffled me with how people discussed the council in mass effect. It was clear that they held no power beyond being symbolic and people still railed at how they were written as if they were capable of solving everything but held back for plot advancement