For example, predation could be a Great Filter. We made it past the threat of predation, but perhaps most life forms in the universe don’t. In this example, life could be pervasive in the universe but it’s optimizing for defensive attributes like speed and armor rather than intelligence. That could explain why we don’t see aliens colonizing the galaxy. With the Great Filter behind us, nothing would be stopping us from colonizing the galaxy ourselves, we just might be among the first to do it.
Putting the Great Filter in front of us postulates that many life forms have achieved our same accomplishments but haven’t colonized the galaxy. It could be because of doom and gloom scenarios like war, aliens, cosmic catastrophes, etc. Or it could be a beneficial Great Filter like enlightenment and the lack of desire to propagate and consume endlessly, or the ability to survive in deep space, leaving stars and planets untouched.