I believe that e-martin answered beautifully. Essentially, literature speaks to the auditory; whereas, art speaks to the eyes. (Simplistic, I know.) Both can touch the soul, the heart, and be inspirational. For some, literature is more appealing because the reader gets to create the image. Art, on the other hand, already displays the image.

In my daily life, I hear about books more than visual art and I run a gallery space...so I have a little trouble accepting the generalization that Tolstoy takes a back seat in the popular imagination to DaVinci. This may be true, I'm not sure, but the relative prominence of one form over the other is not universal. 

Outside of the popularity issue, the question of which form of art is more expressive is an interesting one. Literature seems to be more fully and specifically articulate of ideas. Visual art may be capable, in its way, of representing a more direct translation of emotional experience than literature. 

For sake of conversation, I'd distinguish the two forms in this way, saying that literature offers a greater fullness of expression while visual art offers a greater immediacy of expression. 

I think that people talk about art more because it's a lot easier to experience.  Anyone can look at an image of the Mona Lisa online or in a book and give their opinion.  By contrast, reading War and Peacetakes much more effort.  I think literature can be more expressive because it can use words (and plenty of them) rather than simply having to express ideas through a more limited set of tools.  Even so, people talk about art more because it's easier to "consume."