Two easy options: lack of a sufficiently complex language to transfer ideas and social organization. If a species doesn't get over those hurdles, evolution happ

Two easy options: lack of a sufficiently complex language to transfer ideas and social organization. If a species doesn't get over those hurdles, evolution happily takes another path. And about intelligence: we may not be (or have been) that much more intelligent than certain other apes, but we can learn from other humans from thousands of years ago thousands of miles away. That makes a difference.