If you had space on a card only for ONE of those things:
- Artwork representing the situation, item, etc.
- Flavor text or story text representing item, situation, etc.
Which one would you chose?
They say a picture is worth a thousand words, so from this point of view, a piece of artwork is going to put down more words that you can possibly fit on a card.
For me, I rarely read flavor text. I might be more likely to read it in a solo game, but when playing with others, I rarely have reflex to do so. Also when somebody read me text like in Dead of winter, I kind of not understand what is going on. I need to read it by myself if I really want to understand.
As for artwork, it might not be clear what the designer want to represents with a single picture. But the player's imagination could add up and enrich what is beign represented. You can also get the idea in a glance, without taking time to read. It also makes card easier to identify and spot.
Text might looks easier to write and doing art, but chosing the right words to do the right impression might take as much time as drawing a piece of art.
On BGG, somebody suggested putting the flavor text else where. I have seen this at the end of rulebooks for examples.