There's not enough capital.
Publishers underprint their products. They would like to print more, but they have limited resources and for the mid level publishers, perhaps too large a portfolio.
Like tired fish swimming up a narrow stream, there can only be so much product in the distribution channel. The channel itself is a bottleneck that restricts available fish. As fish enter the distribution stream, I need to decide on my catch of the day. How deeply do I want salmon? Will my supply of carp last through it's inevitable demand?
I could curse the river, and it's narrowness and how it tends to twist and turn giving advantage to some fish over others. If it were only wider, I curse. And maybe deeper. But the real problem is my own. Despite the shortcomings of the stream, the number of fish far outstrips my meager ability to catch what I need. If I only had another line, perhaps I could satisfy all the demand. My problem is I am also undercapitalized, so I have to pick winners and losers among an embarrassment of riches, and hope for the best.
The customer in my shop care nothing about concepts like capitalization, they just want their fish. Some even want exotic saltwater fish that don't enter the stream, but occasionally I'll be tricky and find a way to get those as well, direct from the deep sea fisherman. The most loyal customers will buy from me, if I have the fish they want, but will declare they're heading out to sea to find a catch of their own, if I can't provide.
I sadly nod and wish them well on their journeys. "Good luck and tight lines," I tell them. I want them to be satisfied with their catch, but I know eventually, if I send them out to sea enough times, they'll get a taste of that salt air and they'll stop buying fish from me. The secret to my trade is turning people into occasional fish buyers faster than I turn them into fishermen.