nannyB is right about drying-out.....people still occasionally find awful surprises of this kind in old attics - it's a balance of a stable dry atmosphere and the availabilty of drainage for fluids.
Sorry, just going to return my brekkie.....
Historic reports of bodies being intact need to be analysed carefully - as not decaying was seen as a mark of saintliness, this was among the battery of propaganda claims made when people had axes to grind.
Plus it could include use of sealed coffins and rudimentary embalming that might delay the process.
Then there's very strange environments like St Michan's Church Dublin - do not look at any of the images if you goole this one, you won't sleep for weeks.