My guess is that the great filter is that our galaxy has been too "hot" until recently. Life has sprung up millions of times, but supernovae, black hole radiation jets and the like have cooked worlds pretty regularly. We're out near the edge of the galaxy, so we're in the more distant and cooler section. We may also have been very lucky. Combine that with the limited speed of light preventing us from noticing many of the traces of intelligent that might be far away and faint.