Because being an artist is hard. It is particularly difficult to create realistic representations of three dimensional things in two dimensional formats. Beyond having innate talent and artistic ability it takes the learning of specific sets of skills and techniques to make realistic paintings (and sculptures, etc). Sometimes these skills and techniques can be learned by years and years of trial and error by an exceptionally talented artist with great ingenuity and determination, providing he has the time, money, and resources required to make such advances. Even then a good artist with all those things going for him may only crack into one or two of the dozens of techniques required in making great realistic representations. Usually, however, these techniques are learned from other artists through a long succession of handed down knowledge and careful tutoring and mentoring. Novice artists learn from masters, and fellow artists share with one another what they learned in their own education. Skills, techniques, and technologies are thus added to the repertoire of artists in general as long as information continues to be shared, practiced, and codified.
Sometimes, however, the line of education is broken and the knowledge is lost. This happened in Western Europe during a period sometimes known as the “Dark Ages” when classical Rome collapsed around 400 AD to around 1000 AD when feudal Europe gained full steam. Thus one way of looking at Medieval art is tracking the slow relearning of previously known techniques and technologies in making representational art. They were doing the best they could in the Middle Ages, and consistently making progress. By the time of the Renaissance most of what was lost in Western European art was relearned, and the improvements in technology, communications, and education made possible even further improvements toward making realistic paintings and sculpture. By this time the techniques for creating realistic scenes and figures (such as chiaroscuro, perspective, sfumato, and foreshortening) could be learned by aspiring artists because of a centuries long “conversation” by a long line of serious hard working artists handing down knowledge practical and theoretical. Renaissance artists didn’t just suddenly decide to start painting better portraits, they were using the accumulated knowledge of their teachers mixed with knowledge gleaned from other, previously isolated cultures, mixed with a good measure of their own genius.
Some art historians would also argue that Medieval artists were simply less concerned with realistic representations and more concerned with symbolism, meaning, and storytelling. While I think there is some merit to that, I also believe that any look at a timeline of Medieval art shows a true progression toward better realism and thus shows a cultural desire towards that end.
On a personal note I actually prefer the character, symbolism, storytelling, and simplicity of Medieval art over Renaissance art. The Medieval section is the first and sometimes only section I visit at art museums. And you can bet damn well if I owned that Froissart I’d display it prominently (though it isn’t a painting so it wouldn’t really be hung on a wall).