I presume the secret is the speed with which the pyroclastic flow consumed the site. You get a short period of intense heat which causes charring, then the oxygen supply is gone and the burning ceases. There's enough time to completely destroy a human body, but substantial timbers must survive.

Oh, you must go there! You're a Romanist, for heaven's sake!!! That's like being a prehistorian and never visiting Skara Brae.

Ah, on second thought... I haven't visited Skara Brae yet. Ooops!!!!