They (cats) unfortunately lack the reversible rear feet of squirrels, so down is rather precarious when all your claws face in the "release the hold" direction as you go down.
Not all cats are unable to down climb, Norwegian Forest cats for example are known to down climb head first. [1]
We had the pleasure of housing one temporarily for a couple months. It looked huge but 50% of its volume was wispy fur. Hidden within was a lean petite frame with muscular legs. It was a very different animal from our two other typical mutt house cats, but I can't recall if the claws were organized differently.
This is what my dad explained to me and is further the reason he never helped me down from trees. I'm super thankful for this; I love climbing trees. (And have never seen cat bones in them)
It's just they can't get down sometimes!