How come archaeological finds are so far down? Roman ruins are often under 6ft of earth - why?

This is totally about the situation of the site. Usually archaeological sites are being buried with the later settlements. Usually these sites are founded in strategically good places, then people choose to settle down there many times through the history.

Burying is also a ritual for some societies. For example, some prehistoric people buried their houses and temples before they leave the settlement. This could be a kind of religious activity.

For Turkey, as an archaeologist living in Turkey, I would say, this is also same in Turkey too. As far as I understand you have seen one example about the depth of mosaics in Turkey. But there are many more sites which are deep under the soil too. This is totally according to the situation of the site again. If people choose to live there very long time, the depth of the soil increases. If people leaved and did not settle there again, the depth of the soil decreases.