The filter is The Carrington Event. A little research into the study of stars finds out that the Carrington Event is not an unusual event, but rather a regular part of stelar mechanics. There is evidence that possibly dozens of similar level events have occurred from our own sun during the history of humanity, only technology was at such a primitive state that there was no effect on society until the most recent event. These solar outbursts are on a semi-regular schedule, and most stars produce them from what I understand.
So my Hypothesis is thus:
A civilization has to be able to make the run up from agrarian society all the way to advanced enough to having advanced technology that can survive a Carrington level event, within the window between 3 events. The solar event we call the Carrington event happened at a stage where it did not do serious permanent damage to our ability to advance. The one preceding it was not even noticed, which leaves us with the next one, which some say is late. Based on the degree we have plundered resources from the surface of the planet, if the next Carrington event is as destructive as the first to our modern infrastructure, it may be impossible to recover sufficiently to ever become a interstellar species; Provided we don’t develop to the point that all our tech is hardened against it.
This implies that any given civilization must have the magic mixture of intelligence, political co-operation, and drive to do so to become an interstellar civilization between stelar events. One little error, war, disease, famine, natural disasters, industrial revolution starting with poor timing between events, can stunt advancement long enough to cause the next stelar event to permanently cripple a civilization. Essentially, a civilization has to win the lottery against millions of factors to become detectable. If Fermi is right about how many times intelligent life should have arisen, then there are likely millions of planets with agrarian civilizations scratching out a living on top of the ruins of an advanced culture wiped out by a solar event, and perhaps a *few* who have expanded into their local stelar neighborhood.