You’ll need to find an extended explanation on your own but in essence - religion. The focus is on symbolism, and not on realism as it diverts attention. I remember seeing some examples of paintings made by the same artists for secular and religious purposes, and the secular ones were realistic. But it’s been quite a few years ago and the only thing I remember is that they might have been Italian painters working in the Eastern Roman empire that were recognised due to the messed up alphabet.