One huge difference between the Christian Byzantines and the Ancient Greeks which could account of their disparate art styles is the different values coming from their spiritual systems. The Greek Pantheon was very embodied, very full of ‘salt and vinegar’, didn’t shy away from food, sex, violence, etc. In comparison, the Christian mythology is one of temperance, martyrdom, and transcendence. It makes sense that with their lust for life, the Greeks would depict realism, dynamic expression, and movement in a very visceral and ‘realistic’ way. Christianity however is about transcending this life, so really the realism of this plane of existence doesn’t matter. Interestingly Byzantine iconography starts to resemble the Ancient Egyptian depictions of deities (especially with the halos) which Greek art originated from. Another reason for the difference in style goes back to the other discussion post about the second commandment. Given that artwork was more likely to be illegal, the profession of master artist was less desirable. Iconography probably requires less technical skill that photo realistic rendering, so people ended up doing what they could do. Without a compelling mythology to motivated them otherwise, simpler depictions of saints and martyrs was the flavor of the day.
Again, sorry for the late post 🙁