Here are a few suggestions that doesn’t require the Fermi Paradox to be true, or, for that matter, the theory that we’re the only intelligent or living creatures out there (The Fermi Paradox is in short; since we haven’t heard from any aliens, they’re all dead):

1. Because radio is too slow – communicating over the vast distances of space via radio is like taking your bike to your overseas vacation. If radio is the only way to communicate aliens have given up doing it a long time ago.
2. Because distances in space are huge, vast, enormous – the first radio signal from earth that could leave the solar system (the radio transmission form the Berlin 1936 Olympiad) has today covered about 1 / 20 000 000 000 of the volume of the Milky Way.
3. Because the time we’ve been listening for radio signals, and the fraction of time we’ve sent them out in the universe is about 40 to 80 divided by 15 billion.
4. The search for extraterrestrial radio signals (the SETI-program) does not cover the whole sky at the same time, in fact, very little of the sky has been covered. With today’s technology, we don’t even know what the Milky Way looks like on the other side of the galaxy center, because we can’t really observe it (since the galaxy center is in the way…)