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Oxygen
The plant decomposition in bogs means there is a very low level of oxygen and a high level of carbon dioxide and methane. The lack of oxygen slows down the decomposition process for bodies and other artifacts, and the other gases present in effect preserve them. This is why bodies removed from bogs, despite their age, still can have intact skin.
Salt
Bogs of this nature are often found near coastal areas, where the constant moisture in the air facilitates their composition. Salt from the sea seeps into the ground and into the bogs. Since salt is a natural preservative, the organic matter in the bog is preserved in the same way sailors used salt to ensure meat stayed edible for longer during the age of sailing.
Humic Acid
Peat, a common component of bogs, is organic matter as well and decomposes over time. New peat forming near the surface forces the older material down, where it rots, creating humic acid. This humic acid works on organic matter in the same way as pickling vinegar.
Temperature
Even during the height of summer the internal temperatures of bogs in certain regions will barely rise above zero degrees Celsius. At these low temperatures bacteria are unable to reproduce, so there are no organisms to facilitate the decomposition of a body.
Sphagnum Moss
Sphagnum moss commonly grows in bogs and extracts calcium from organic matter, which means any bacteria, or other organisms, have nothing to feed on and so leave the body alone.
Time
It is important to remember that how long after death bodies have entered a bog has a huge effect on how well preserved they are. Time is important since decomposition is a irreversible process, so if a body had started to decompose and then been put in a bog, the decomposition would continue for a while afterward. This is why there are varying degrees in the state of the bodies removed from bogs.