SteveC said:
What it sounds like is that this is a relatively new issue, and one that is getting worse. Am I correct here, or is it just something I never saw before?
The following is just the gist of many similar threads, so bear with me

:
Well, I heard it was starting around the release of D&D 3.5. Game shops couldn't sell D&D 3.0 stuff anymore. Without money from sales, they didn't order new stuff or only very small numbers. Many game shops died, and if I remember correctly, alone during the last year, one third of all game shops vanished (that's how I remember it; someone with exact numbers may correct me). This means that the number of sales points for RPGs that are dependent on game shops decreased dramatically.
Most customers who go to a shop in order to find a product do that exactly once. If the product isn't there, they go somewhere else. With less copies ordered, even product that would normally sell did not have any customers, because the latter used other channels to get it. This means: no reorders. Distributors didn't get rid of the stuff and didn't want to order much from RPG companies anymore. The main distributor of a big number of small to medium companies finally got bankrupt, not paying said RPG companies. For many RPG companies, this was the end.
Distributors also start with the "lean management" model. They keep only a very small number of product in stock. If the product unexpectedly sells well, they cannot fulfill reorders in a timely manner. This means more lost customers.
Companies like amazon don't like product that is not available from distributors in a timely manner. Amazon has small profit margins, but if customers complain about overdue deliveries, they usually hand out $5 compensations. Guess whose product will get penalized by surcharges, no discounts and, as final measure, be kicked out of the catalogue.
EDIT: As I wrote it, it looks as if I made 3.5 responsible for the game shop death; in reality, this was only one small aspect among others, like a decline in the important card sales.