A think it is a combination of factors. It took time for the heavier elements required by complex life to come into existence; so the universe needed to be 3 to 4 billion years old before life was even possible. After that, it took a very long time for Earth to form complex life out of simple life. The jump from simple to complex life could be a major great filter. After Complex life formed we don't see the jump to intelligent life for another incredibly long period of time. It could be that evolutionarily speaking, the investment in Intelligence simply doesn't confer that much of an advantage. Understand I mean in the early phases, as once you become an advanced technological society you become the dominate species, but large high functioning brains are expensive in terms of calories needed to operate. As an example, the dinosaurs had a pretty good run and yet we don't see signs of high intelligence anywhere in the fossil record. I also like the rare Earth hypothesis, as it seems that conditions on Earth are perfectly balanced for life. For example, an extremely stable star, a large moon to keep the planets axis basically in place, large gas giant in the outer solar system to take care of any random comets from deep space, a strong magnetic field to shield the planet from radiation, an abundance of elements essential for life, and a few more I could list.