Standards for Art vs. Standards for Literature

I've been thinking about the different standards for art and literature. In the post modern art world artists are constantly being challenged to push the envelope. And, as far as I can tell, one of the goals of artist's now is to create something "new" and often times "new" is associated with "shocking." In the case of art there appear to be no real boundaries or taboos limiting creativity and execution. So I'm wondering, why do there seem to be different standards for literature? Pornography is not new, and although it is still shocking to most, it is not a new concept or form of expression. Why are the standards for art and literature different? Is it because when dealing with literature the tendency is to impose more preconceived notions? Is it because when creating our own visual images the intellectualization of sex becomes more erotic than if we are just handed the images on movie screens and magazines? Even elements in some scenes of Blue Velvet that could be seen as pornographic, the mechanized expression of sexuality shown in Blue Velvet resembles the descriptions of intercourse in Tropic of Cancer in the sense that the purpose of the sexual element in both the film and novel are not meant (in my opinion) to "arouse" the reader/viewer. Still, somehow Blue Velvet retains the classification of "murder mystery" and Tropic of Cancer is labeled as pornographic. How did that happen? Again, I think that the differentiation, in this instance, between "high" and "low" art depends a great deal on determining whether or not the film/novel is exploitative in nature. But the standards for judging the two mediums of expression are different. For one reason or another the literary standards for judging and classifying seem outdated, and because of this, a whole category of literature is pushed to the side. A category that many would argue has an abundance of "high art" to offer, and in the face of so many sexually exploitative, "low art" movies and novels widely available to the public, why should pornographic literature not receive its dues?