My “boring” “explanation”: the number of alien intelligences is unknowable and irrelevant because interstellar travel and communication are both near as damnit impossible.
Yeah I wonder if this isn't the simple (and sad) truth. Maybe every intelligent species discovers that the laws of physics are unavoidable hard limits and eventually decide it's a pointless waste of energy to endlessly broadcast signals to other stars and galaxies that are moving, ever faster, further and further away.