So you are asking why the "dead god" did not decompose, is that it? If that's what you are asking, the reason is not because his hands are like leather. You need to look just a little farther on in the story. What the author says there is this:
The room was shut, hot and dry—no doubt that had kept him as he was.
So this is what has kept him from decomposing. He has essentially desiccated and become a mummy. That is, I think, why John knows that he will "fall into dust" if John touches him.
So this body is one that has not decomposed like all the others because it was in a hot and dry room where it could just dry out like those Inca mummies that got mummified by being up on dry mountains.
I've provided a link that talks about how a body can become a mummy instead of decomposing.
Thanks! This is so much help. It was the only question that I couldn't find on my study guide!