The problem is not a lack of naturalism, it's obvious mistakes in the way the naturalistic poses are attempted. Many of Rublev's icons have obvious mistakes in the way joints are painted, for example - but not all of them or the exact same thing; it's not a style, it's simply a limitation of his skills. Many later painters who were inspired by him have corrected this mistake, not sought to reproduce it.

Not to mention, Rublev lived at the end of the Medieval period, and well into the Renaissance - the period where painterly skill in Europe was revitalized.