Why?
Why are joyous 4E fans not buying the 3PP product that is out there?
If 4E fans were buying Mongoose would sell to them.
Mongoose tried with ... what was it, a Quintessential book with shady mechanics and a kind of odd and mostly systemless new setting?
The Quintessential product line was already pretty debased by late 3.5, so I'm not too surprised about its lackluster performance. (Additionally, did they even advertise it? I didn't hear about it until well after it came out, and I'm on several boards every day. And when I did hear about it, it was generally along the lines of how bad it was.) Wraith Recon, from all accounts, had pretty much no mechanics and was pretty lackluster. Again, not something I'd use as a predictive example.
You are saying that the market is dead because there is no one interested in offering supply. If there was a demand, the supply would emerge. My point is, the demand is not there. You can not blame companies for not making products that don't sell.
That's ... not the way it works. Demand does not create supply, generally. It can create
opportunities, but someone has to take those and run with them.
I see no reason why an excellent setting along the lines of Arcana Evolved, with interesting new mechanics and a new flavor, wouldn't sell. There are a lot of people anxiously awaiting 4e Earthdawn, and a vocal population waiting for a 4e(ish?) Freeport. You can't blame fans for not buying products that don't exist.
Both Mongoose and GR have made it clear that they are staying with the lines they see as the best business choice. Why is M&M a better seller for GR than 4E? Or, at least, why are they placing their bets that way?
GR and Mongoose have both made it abundantly clear that selling their house system is a much better bet than selling for a system that's not theirs. Pramas has written pretty eloquently on this, as have folks from Paizo. Simply put, for a publisher, it's way better to control all aspects of your business. (See: the 3.5 apocalypse for third-party publishers.)
If a company can afford to do so, it's much safer to become a first-party publisher - or else to make products which can be adapted to new systems quickly and easily. (Like Goodman did, and like Paizo still does with many system-neutral products.)
Why doesn't someone else emerge to replace these companies?
Well, for starters, because - like I said - the market itself was shot well before 4e was announced.
Frankly, even for third-party publishers, the market is
way different than it used to be. White Wolf got into the mix early by releasing a monster manual before WotC's own - something that won't happen again. Green Ronin made a splash by making (IIRC) the first available modules for 3e. Not so here, either.
The economic situation is far different, too. Third party materials weren't selling well before 4e was announced, so the distributor network has cold feet regarding them, new edition or no. There are new demands in the print industry involving rising costs and changes in bookstores. The quality bar is much higher, now, too - early d20 materials could sell with just a color cover and staples; nowadays fans expect better art and better editing, even from the smallest third parties, increasing production costs. Fans are more demanding, too - look at the gap in production values from Relics & Rituals to World's Largest Dungeon, Wilderlands, and Ptolus. And yes, the late GSL hurt a lot, too.
Most of the companies who were still around at the end of 3.5 were either some of the first out of the gate with quality products, or else got a leg up in some other way. Green Ronin was one of the first. S&S was up there, too. Mongoose, as well. (I have no info on Necromancer - sorry!) Goodman's products are probably among the least expensive to produce, so they're fine regardless. Malhavoc had Monte Cook and financial ties to White Wolf, not to mention a well-earned reputation for sound products. Paizo was the publisher of Dungeon and Dragon - a huge advantage on their competition. These are generally companies who were able to either build capital early on, or else had a leg up somehow. They could compete with WotC on quality, and their success showed it.
You can't just look at 2000 and look at now and say, "Well, why the heck isn't this happening again?" because the situation is frankly not even remotely similar - especially for a new company.
My point is that 4E players do not buy (not enough). If I am wrong, someone will sell to them. Thing is, I'm not wrong.
Again, complaining that people aren't buying things that aren't being offered is ... shady.
-O