There can be a number of reasons.. The site could have been buried by humans (i.e a burial site), or have been built over, some sites may have had numerous structures built and knocked down and then built on again.. The weather may have blown or deposited debris/sand over the site and this accumulates over years into deep soil/sand. A rise in sea level or a flood at some time may bring silt and mud over the site and buried it this way. Volcanic action

I would like to add onto the answer already posted. Artifacts become buried in just those ways stated above, but also in much simpler ways. Imagine if you lost your watch in the backyard and never went back to retrieve it, within just a couple of weeks it would be overgrown with grass or weeds, and after a couple of seasons it would begin to blend into the ground even more. This is because the shifts in the environment make the ground change, snow melts and deposits sediment when it washes into drainage diches and sand blows very slowly until it has covered it over. These are slow ways that artifacts can become buried. Tels, such as those in Israel are formed by direct influence from people. If I build a hut and then add to it as time passes, and later tear it down in favor of a new one, these bits of building material crumble onto the ground and get stomped down until they have actually added to the height of the surface. Tels are many ancient cities one on top of the other and they tend to take on an odd shape (which is how many archaeologists can identify them); a house was never bulldozed like we do today, just knocked down and then replaced with new material. Pottery which is a very common material to find on an excavation (especially in the Old World) happens to be on the ground everywhere because of its tendencie to break, and since it was cheaper than many of the other vessels (silver, bronze, gold, glass) it was just discarded (I should also point out that pottery made of nicer "fabrics" is less common because they would try and mend it--but that's another topic) and since they had no driveway garbage pick-up they discrded it where ever was convenient. Sometime what is called a midden is found and these are areas comprised of rubish, which did manage to find it's way to a designated discard place. I hope this helps to clear up the question, just remember that earth is always moving in ways affected by people and by the environment and the dust always settles somewhere; if there is no one to clear away the dust as it settles it dissapears beneath it.