For book/ebook authors, publishers, & self-publishers
The biggest challenge for most book authors is very simple: They don't believe in their books enough to spend some time every day marketing their books.
I'd imagine the answer is fairly simple. Being noticed. The world is flooded with books. Quite often books by indie authors are of a higher standard than those pumped out by commercial publishers. It mostly comes down to marketing. If a book is pushed hard enough and enough money is spent then the you'll probably have a best seller. The actual quality of the writing will be secondary. Against all this you have the independent authors desperately trying to be seen and get some traction.