There are fewer established, print companies doing d20 stuff. In fact, of the top d20 companies I don't think any of them were around 6 years ago. I could be wrong, though.
Way back when d20 first came out of the gate, I remember Ryan Dancey saying that he was surprised that so many companies had jumped on to d20 so aggressively. He had expected a much slower build up of volume. Instead, we saw a rapid flood of material.
The one thing I've learned about the gaming industry is that, almost invariably, if anyone has a whiff of success with an RPG product there'll be imitators. The costs to produce stuff are low enough that you can risk a few books on what looks like a hot topic. d20 definitely fell into that category.
The problem is that of all the systems out there, d20 is the hardest to design for on two levels.
First, it's the most mechanically precise game on the market. Even more importantly, its fans are aware of the mechanics and generally seek mastery of them. Unlike the hordes of rules light games out there, D&D rewards you for learning the rules.
The problem is that, as a whole, the industry didn't have anyone trained to write d20 stuff. Early products (mine included) are full of rules gaffes. However, this never really got any better. The industry as a whole wasn't used to producing a product like D&D. If you look around the market, the staggering majority of RPGs use simple rules sets. d20 presents a mode of design that most writers in the industry aren't comfortable with, even 5 years after its release.
This dovetails into the second point - most publishers and game writers simple don't get D&D. The staggering majority of d20 books aren't bad, or horribly written, they're just pointless. There are literally hundreds of titles out there that don't offer any real, compelling reason to buy them. My sense is that, in a lot of cases, publishers just solicited ideas from freelancers and published the ones that sounded good. Very few, if any, companies had long term plans for their d20 publishing schedules, or any predictions for how d20 could evolve and how to respond to likely changes as the market matured.
The problem I see now is that WotC is leveraging its advantages to produce designs that are consistently superior on every level to third party stuff. For a time, d20 stuff could compete with WotC. Wizards had the same problem with building up a knowledge base and methods for d20 design. They also produced softcover, black and white books just like d20 companies. Now, neither of those are factors any more.
So, IMO that's why you see a slowdown in d20 products. There's always been a backlash - a lot of people in the industry hate d20, and a load of dysfunctional gamers will hate whatever's popular. But that's been there since day 1. I remember people claimed that d20 would be a colossal failure. When the books started selling, those same people predicted that d20 couldn't handle other games or settings. When they were proven wrong again, they predicted the death of d20 and have done so now for 4, maybe 5 years.
Just dreaming!
