There are lots of theories for this, but the one I find most persuasive is that we became bi-petal at some point in out evolution (we started walking on two feet). Now other species do this, but not exclusively. The reason this is so important is it freed up our hands to explore the world around us. These became incredibly sensitive to touch, heat, pain etc and the brain needed to develop to process all this extra information. This simply didn’t hapen in other species. Even the great apes, our closest relatives, still use their hands to get about.