Interesting question! There are so many reasons. There are also many types of art. Art can be the work a child brings home and hangs on the refrigerator, a photograph, a sculpture, or even an installation. I interpret "art" to be the objects we put into an art museum.
First, most people are afraid of others thinking they are ignorant about art. Most of us don't have the vocabulary to talk about a piece of artwork. Words like value, contrast, hue, balance, aren't a part of our everyday vocabulary. We are unsure of genre and medium. "Contemporary", "minimal", etc., what does it all mean? I would be uneasy speaking to a doctor, lawyer, or other professional if they used vocabulary that is particular to their field and I was unsure of what they were talking about. So you need to learn some vocabulary specific to the topic. If you don't, you feel you shouldn't offer an opinion because you are out of your league.
Much of this discomfort comes from the fact that some people have never visited a museum. The first step to talking about art is being comfortable around art. It may be that people are intimidated, unsure of behavior, cost, etc. If you have not visited a museum you might not feel you can have a conversation about what is hung on the walls.
Most people don't consider art to be part of popular culture in the way that movies are. Movies are in theaters, on television, on video tapes, and DVDs. They are in our homes. People will go to see two or three movies a month, yet hardly ever venture into a museum. We can define why a movie is good, why we like it, if the story makes sense, or if the characters are good. We even have someone tell us if a movie is good or bad, giving us a thumbs up or thumbs down. Not so with art. It is easier to look at a movie and define it—mystery, comedy, love story. These terms come easily to us. A painting or sculpture that makes us say, "What is it?" doesn't make for easy conversation.
As an art educator I can continue teaching the value of art and how it enriches our lives. I can teach my students that they can use "art" vocabulary and feel comfortable doing so.
—Kris Wetmore, Visual Art Teacher, Cooperative Arts & Humanities High School, New Haven Public Schools