I think people like period pieces for the same reason people like reality TV. It is a more acceptable form of social escapism, especially the deeper and more historically accurately you go.
Consider the following critically acclaimed shows: Mad Men and Dowton Abbey. Now consider these less acclaimed counterparts: Honey Boo Boo and <Insert any Kardashian show>. Each of the four exist to tell us about a segment of social life. Some are built around the upper class and the constricting mores and folkways of the upper crust. Some drift the opposite direction, lending themselves to the baser side of life and the trainwreck that is a particular segment of the American population.
Just as alcohol is preferable to cocaine and coke is preferable to meth, so lies the type of shows we watch. Social escapism is the destination, and though the paths to get there are divergent, the destination is the same.