Perhaps the laws of physics dictate that there must be one civilization in the universe. Not less, not more.
Why? Because life, or consciousness, can only exist if the laws of physics are consistent.
Let me provide an example description. In the beginning, there was human consciousness. Nothing else existed. For example, there was no Sun.
The latter statement should be read as "Sun was not". So I'm not saying that the Sun didn't yet exist. What I'm saying is that not even the concept of "Sun" existed. "Sun" wasn't present, "Sun" wasn't absent. It didn't exist, it didn't not-exist. It just "was not". It was like "undefined".
But then, the consciousness looked upward, and observed a yellow disc.
From that moment on, the number of possible universes was greatly reduced. Until this disc was observed, many universes were possible: ones with a yellow disc of course, but also ones with a green square, ones without any such thing, ones with 42 dodecahedrons circling each other, and so on. But as soon as the yellow disc was observed, the number of possible universes was reduced to those were a yellow disc could exist.
And so on for all other observations. Each new observation reduced the amount of possible universes we could be living in. One could say that human consciousness created the known universe, just by observing it.
Now imagine that the universe contained two, or even more, consciousnesses that independently observed the universe. That's impossible! They would be independent, so they would be creating different universes. Yet they would also have to be in the same universe we are in if we were ever to observe them. These two requirements are incompatible.
Therefore, we are alone. We must be. Because there's only one law of physics, which is the law of consistency.