An alternate "simplest" explanation (though less likely) is that we are first.
Just curious but why do you say that? We have no clue how likely intelligent life is to evolve. All we know is that it has happened once, and it took 3.5 billion years from the formation of the first like on Earth. Suppose that this was very much faster than average and the the mean time for intelligent life to evolve (once life itself has started) is 30 billion years? Such a long time would hugely reduce the number of intelligent species since you need a very stable environment for a long period of time and even then you have to get lucky.
Trying to quantify what you don't know is a mug's game...in order to be able to do it you really need to know what you don't know. If anything I would argue that there is, perhaps, some weak evidence for intelligent life being rare: travel might be hard but radio is easy. We have not heard ET's broadcasts which would suggest perhaps that there is no intelligent life nearby (or they use some technology beyond EM waves).