I have a very basic theory:
Perhaps the genesis of life needed (or took) a certain amount of time to evolve or transpire, so that would explain why there may not be civilizations that are far more advanced than ours. Maybe we are slightly ahead of most. Maybe the building blocks of life have always existed throughout the universe in old galaxies and asteroids all along, and the final ingredient (time) needed to be added before it could begin the process. Then this “fertile” state was spread by asteroids to newer planets.