Why do toddlers go up and down the stairs on hands and knees ?

My son is 17 months international age currently. He started walking early up by 8 months like my parents say I was and very active. YET he can't be walked or led up and down stairs on his feet. He will let go of your hand..... just so he can turn around backwards to go downstairs looking backwards and stepping down carefully with his hands on the dirty ground or either crawl up the stairs down on all fours like some kind of animal. How can a kid walk, run, jump, and skip literally everywhere they want to go even climb and fall off of where they were placed when you don't want them to YET can't walk stairs on two feet?!