Because medieval art and renaissance art are doing different things. Medieval art was more about adornment (eg the incredibly intricate art of the Book of Kells or the Lindesfarne Gospels) and about symbolism (eg most manuscript illuminations). Renaissance art came about as part of a movement back toward realism and set expectations of "good" art as being realistic for the next 400 or so years. This is why medieval art was derided as being "barbaric", or "primitive" or just plain "bad" in the nineteenth century. It was not until the move away from realism in the later nineteenth and early twentieth centuries that medieval art came to be seen as rich, complex and skilled despite not being realistic.
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