Seth Godin uses goji berries as an example here but the same can be said for Network Marketing products...
I think the reason you don't see a lot of kids on unicycles is that they don't come with training wheels.
[in the old days, I would have stopped there, making my point enigmatically]
Alas, most products that are hoping for growth in new markets forget to offer training wheels as well. Things like goji berries, Linux, tattoo parlors and yes, book publishers.
Unicycle training wheels wouldn't even be that hard to do. Neither would a free sample pack of goji berries at the checkout.