Why is epidermis considered a protective tissue?
Because it is the border between the environment and our body and its main purpose is as a physical blockade.
The epidermis is considered a protective tissue because its prime purpose is to form a physical barrier between the outside and inside of the body.
On the top stratum (layer) of the epidermis, the cells are dead, and so have no physiological function besides to form a barricade against the outside.
This isn't totally true as the epidermis also serves to facilitate water moving in and out of the cell, though this could also be seen as its duty as a barricade.