Western art music

Why do you think we discuss Western art music (what most people would typically call Classical music) according to historical periods but we do not generally discuss other types of music, such as folk or ethnic music, in this way? Please note that there are two versions of "O Fortuna" on the Chapter 1 playlist. The first is a re-creation of a Medieval period song. The second is a twentieth-century interpretation of the same words by the early twentieth century German composer Carl Orff. Which do you like better and why?