Because they don't want to pay for them on sites like iStockphoto that already exist to give photographers a way to market/sell their products?
Because istockphoto doesn't have anything resembling "news" photos. Whereas Flickr's got lots of photos with real people in them.
I've used istockphoto extensively. it's all models, references, icons etc.
If I need a decent picture of the Mayor of Chicago at a recent festival, there might be 10 on Flickr.
There won't be any on istockphoto.
Because they don't want to pay for them on sites like iStockphoto that already exist to give photographers a way to market/sell their products?