Voted for
- Intelligent civilizations are too far apart in space or time
- We are not listening properly
- They tend to isolate themselves
- They are too alien
- Earth is deliberately not contacted
- Other explanation (explain in comment)
https://www.isaacarthur.net and his YouTube channel has discussed some other possibilities too. There are lot of videos. It will take some time to find the exact ones you deed. But all of the content are amazing.
This is the best poll I have seen in weeks. Thank You!