A few possibilities:
1) Aliens are technology snobs. They are only willing to contact civilizations with a certain level of technology. We are barely the equivalent of some third world country to them. They want to avoid us as all costs because we are so backwards.
2) Xenophobia. Intelligent species tend to not like other intelligent species for whatever reason. So even if they detect an alien species, they stay away from each other.
3) They don't know we're here. Perhaps they have moved past radio communications and now rely solely on some sort of FTL communication method that we have yet to discover. In doing this, they become invisible to us, but the flip side might be true. They might have been using this communication system for so long that it just doesn't dawn on them to contact us.
And one last one but likely the most probable:
4) We just weren't listening. Think about the history of human communications. How long have we had the ability to receive radio broadcasts? Around 80 years. So if an alien civilization sent a "We're Here" message to us a hundred years ago, it would have zoomed by us without us responding. Maybe they tried contacting us and we just didn't answer to they moved on assuming there wasn't intelligent life on Earth.