I think its probably a time vs money thing. I can only really talk for myself but I imagine most artists do it all for fun and to relax and inspiration comes from out of the blue. To suddenly be expected to just produce pictures is pretty hard (even if I were being paid). Its a huge commitment if its more than a few images. I still need to work so to spend time illustrating a book that I will very likely not make me a penny doesn't make any sense as I would be better drawing something that I want to with no pressure.

That said I illustrated a book not long ago as a collab as the story was just up my street and it got to No 1. bestseller on the kindle (its now about 76, its here if your interested :- http://www.amazon.co.uk/Crackly-Bones-ebook/dp/B004VWXR6W/ref=zg_bs_362230031_73) even then its made next to nothing in any financial way, which is not a problem as I did it for fun.

I always wanted the opposite for someone to write a poem or short to go with one of my images. I am illustrating a couple of others at the moment for the same guy. But it is a unique situation. I went to him with a couple of ideas for short stories that I would have liked to have written if I had the skills ( I could see the pictures in my head) and he loved the ideas and other a few beers we fleshed out the plot and bingo he is writing what I could only visualise.

I think the upshot is if an artist reads something and it rings their bell so to speak they will fall over themselves to be the one to illustrate it, if it doesn't then even with good payment they more than likely won't want to know.

 

I could of course be completly wrong, I was once before LOL