This is based on what I've read and discussed with doctors, please don't take it as facts. It's just my understanding.
The scrotal skin is much thicker and the penile skin is MUCH thinner. The scrotal skin can be scraped/shaved/cut into the desired thickness, but the penile skin cannot be made thicker. Both are stripped of their outer layers and scrapped of hair follicles as necessary, this procedure should be about the same.
some people have a penis that sits high above the pubic bone. This means that for a 7" penis sitting 2" above the pubic bone, you'd get 7-2-1= 4"of depth + give or take some, depending on the stretching, whereas with a scrotal skin graft no such problem exists, it can be placed anywhere..as far as I know, the penile skin has no self lubricating part, but the scrotal skin MIGHT (if not lubricating, at least tissue that can feel moist?)
If I'm wrong about any of this, please let people know. this is just my understanding. I hope there isn't any wrong information in there, but I can't promise anything.