It's pretty straightforward to me.
You can glance at a piece of art -- or even a thumbnail -- and tell whether or not it's worth a closer look.
On the other claw, you can't tell if a story is going be worth your time until you've already spent a significant portion of that time.
The "entry fee" is much lower for art.
This isn't just the furry fandom, either. It's part of internet culture. People make careers out of web comics, and become fairly well-known; prose fiction on the web doesn't get the same audience.