Psion said:
As someone else alluded to, if there is a reason behind resistance to acceptance of third party publishers, it's lack of market penetration. Everyone who plays D&D knows who WotC are. Not everyone knows who else to look for... or look out for.
I think another important reason for consumer resistance is that most 3rd party publishers have refused to effectively take advantage of open gaming. Many companies still do not clearly delineate the open content in their works, or else release crippled OGC [1]. And most companies ignore perfectly good existing open content in lieu of reinventing the same wheel over and over again [2].
Because of this, 3rd party books generally have only a fraction of their potential value, because they are rarely referenced by other books. In contrast, Wizards has a large (but closed) network of new rules outside the core rules, and their network gets stronger with every new WotC book because these rules all build on each other. Libris Mortis might refer to a feat in Complete Warrior, that might mention a monster from the Book of Vile Darkness. All these cross-references reinforce each other, add new value, and make the network stronger.
Third party publishers by and large do not have such a large, shared network of rules to draw on – though they could and should. For example, I really like the Tome of Horrors, but imagine how much more useful the ToH would be in a world where the ToH was a standard reference work for all other d20 publishers; where other d20 publishers freely drew on this resource to populate their own books with new monsters. But instead, we get 10,001 minor variations on the same monster concept.
[1] Necromancer Games, in contrast, was a shining and early example of how to clearly mark open gaming content.
[2] Green Ronin and Bad Axe Games have been particularly good about using good Open content, as seen in the Black Company Campaign Setting and Grim Tales.