A sliced loaf of bread will go stale quicker because more of the bread is revealed into the atmosphere where there are carbon and other elements in it which affect the surface area of the bread and the inner parts of the bread. It would take a shorter rate of reaction to get into the mid center of the bread than an unsliced loaf. Additionally, the dust and things in the air have to go through a tough surface of cells before it reaches the main part of the bread.
