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 movment 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. The Post-Impressionists, for example, were inspired by the way early medieval Romanesque art suggested things without depicting them realistically. Picasso was also a great admirer of medieval art.

Let's take some examples. Here are some serpent beasts from the Book of Kells:

They are not realistic, but are they "badly drawn"? Of course not - they are a remarkable exercise in precise, intricate form and decorative art. Or take this figure of a man from the same work:

Again, this is not realistic, but anyone would be clear that this is a picture of a man. The emphasis is on the symbolic presentation 0f a person of authority in a way that fits with the decorative scheme of the rest of the page and the whole work.

Other medieval art is more concerned with symbolism than just decoration:

This illumination of St Dunstan is not trying to depict him realistically. To begin with, Dunstan is not likely to have sat as his desk to work on a book while wearing his full bishop's regalia, complete with his mitre on his head. He's depicted this way to tell the reader who he is. Nor would he have been writing on a bound book - it's depcicted that way to indicate that he was the author of a famous book. And the book's pages are turned to the viewer so that the text can be read. This is to show that the book in question is his Commentary on the Rule of St Benedict. The emphasis of the picture is on who the figure is and why he is important, not on realism.

Other medieval art is used to tell a story and give a sequence of events, a little like a modern cartoon strip. In these the depiction emphasises who each actor in the story is and uses a language of iconography to help the viewer understand what is going on:

Here is a sequence from the Bible story of Saul and David. The arrangement of the figures indicates the sequence of the action and a stylised system of body postures and hand gestures indicate what is happening. Figures who are gesturing are usually speaking and the kind of gesture can indicate what they are saying: presenting someone else to someone of authority, asking a question, teaching etc.

Medieval art could be realistic when the artist chose. Take this carving of the wife of the lord of Naumberg from Naumberg Cathedral for example:

But most medieval art was not setting out to be realistic - it was decorative, symbolic or both. As a result, it was highly stylised and didn't come to be appreciated fully until more recent times, when the idea that "good" art was realistic art changed.