It's just that the sister republics tended to copy the political system in France. So when the Batavian Republic was created it followed the Revolutionary committees of the Committee of Public Safety. When the Italian client states were created they followed the model of the directorate. And when Napoleon changed to a Monarchy, he felt that the French client states should be monarchies.

On a practical level, overturning and doing fraud on elections costs more politically that imposing a king on a people who were accustomed to being ruled by kings.