One of the producers (Darabont?) said some time ago that, rather than decomposing quickly and completely, the "older" zombies become "mummified" and therefore can be around for quite a while.
If he said that, then he's operating from a belief I would expect from a 2 year old, as exposure to the elelments (sun/radiation, insects, larger animals, etc.) does not lead to a mummified state, which is the reason bodies discovered out in the open after a certain period of time are reduced to bones.
We know this is fantasy, but excuses like Darabont's (trying to apply a real world rationale) do not help the credibility of the series' zombie population matter.