We are looking for intelligent life broadcasting to us using a method we are currently able to detect.
There are a lot of factors reducing our chances:
If you look back before we discovered electricity, we would have no way of receiving the signals we are currently broadcasting at any distance! The luck involved locating a civilisation of appropriate age to receive this signal would be tremendous. Presumably, soon we will discover a new technology with which to communicate, and again the clock will be reset on how advanced the recipients of any message would have to be.
Our thirst for knowledge is pretty unique on Earth, as is our egotism. Aliens would need to be advanced enough to pick up our signal and care enough to broadcast one back.
Knowing what to look for is difficult. We see pulsars broadcasting quickly across the universe and presume they are a natural phenomenon, as we do with meteorites. Maybe they are communicating and we are missing it. For example, once we invented the light bulb, we might have thought to look for light to detect intelligent life. In truth, we have made our lights more efficient, broadcasting a narrower spectrum and reflecting the light going in unuseful directions, making them harder to detect.
The number of alien races. If there are a lot of alien races, advanced enough to communicate, why would they bother contacting us if we are technologically inferior? What if one of these races goes around destroying all the other advanced races it discovers (Look at human history!)
We have enough threats on earth already. Maybe any intelligent beings are content enough with what they have or wise enough to predict how hostile we are to each other and therefore go as far as to hide their presence from us until we reach an appropriate stage of development.
There still exist tribes living in isolation in the Amazon, who throw spears at planes. We have decided to leave them to it, maybe aliens have decided the same with us?
Maybe the natural formation of the universe is such that at a specific distance there is a block of some kind. Imagine yourself as a cave man. Trying to communicate to the outside world might consist of riding a horse to the furthest corner of the land you were born on. After that point you give up. The same once you work out you can traverse the sea and map the planet. Then you work out there is an atmosphere which changes as you go up with stars beyond. Maybe there is a barrier preventing communication.
Maybe at some point in the past, our planet was seeded and they are observing us as an experiment which they don't want to influence by interacting with.
To summarise, there are loads of reasons why we might not be receiving alien signals. My opinion is that it is ridiculously egotistical to assume that we are advanced enough to detect them because any intelligent life will be similar to us. I can only hope they are smarter than us and aren't bothering. Sorry for the formatting