It's the distance and speed limit of light.
If an alien scientist from Andromeda (the closest galaxy from us) points its telescope at Earth, it will be seeing dinosaurs. It will take another 2 million years from today before any Andromedan will ever know that there is an intelligent civilization in our solar system.
So that means the only way any alien race could detect us, is that they will have to be inside our Milky Way galaxy. But our galaxy is 180,000 light years wide, while our civilization is only a few thousand years old. So that narrows it down to only a tiny spec of the galaxy that can see human civilization from a thousand years ago.
Imagine a planet 1000 light years from Earth seeing today how humans lived 1000 years ago. Will they try to send us a message and assume we would have the technology by now? Maybe they will. Or maybe not, after seeing what we do to fellow humans during that time. Will they try to contact us? Even if they did, their message won't arrive until the year 3018.
How about we narrow it down further. An alien looking at us from 73 light years away will be seeing World War II. Will they send us message? Hmmm, this Hitler guy looks like he could win. Let's point the telescope somewhere else.
When you narrow it down to 30 light years there's only a handful of exoplanets that can be inhabited. Too few are that close. The rest are too far.
Assuming there are a handful of alien races that are close enough to see us in our recent history, you still have to narrow it down to a specific alien race that has progressed far enough to solve the speed & distance problem (i.e. wormholes, etc.).