Snakes shed their skins and rabbits don't. That is a difference between snakes and rabbits that explains why we see snake skins and not rabbit. What difference between reptiles and mammals explains why we find entire mammoths intact, with skin like rawhide, but no dinosaurs in a similar condition?
Reptiles have very fragile skins, while pachyderms have tough skins riding over a thick band of fat. Also the dinos mostly were swamp dwellers, while mammoths dwelt in mountains and plains.