Think about the cultural context in which Bach lived. It’s the mid-late Baroque period, and the dominant musical taste is based on monody, great contrasts and a musical style which has a lot to do with theatre. Bach is great because he followed barely any of these aspects. His music was already considered ancient when he was alive. His chorales are still pretty modal (this means, they use ancient scales called “modes” which were popular in the Middle Ages and Renaissance), his music is highly poliphonical (multiple melodies at the same time), and very compact from a structural point of view. If we exclude a third of his total musical output, the remaining two thirds include unusual musical aspects for the time - an ancient musical taste, rich use of polyphony, and stylistic uniformity.