hi! not discouraging any art historians from contributing here, but FYI, there's a related section in the FAQ that may interest you
They didn't, this is entirely the wrong question to ask because it assumes that closer to photo-realistic inherently makes art better, which is as wrong as you can get in regards to art criticism.
Visual art isn't about photo-realism, it's about the meaning the artist is attempting to convey using the art, whether the techniques to convey closer to photorealism had developed at the time is a legitimate question, but the question in an of itself is just bad art criticism.
I'll give an example just to make the point, would you argue that Animaniacs had a worse animation style Final Fantasy: The Spirits Within? The latter was very obviously much closer to photo-realism, but had issues with Uncanny Valley, and furthermore the latter's ability to deviate more in terms of form allowed them to animate it more expressively.
You should rephrase your question in regards to what exactly you were trying to get at with it and remove the categorically wrong idea that Middle Ages art was inferior when it merely had a different objective.