I think you have to lay a lot of the blame for this at WotC. They put the game out via OGL and shortly thereafter the OGL champions were gone. There wasn't a centrally coordinated effort to realize the potential of the OGL, an effort that should have been led by the source of the licence and base rules, WotC.
Disagree.
They okayed making a lot of IP OGC with the Tome of Horrors. They included OGC from at least one other publisher in the MM 2, and did so in a very cool way to boot. Lots of the psionics, epic, and deity information was included in the SRD shortly after release. When UA came out, again, it included tons of OGC.
They dropped the ball on utilizing the best of the OGC out there for a new version of the game -- which is perhaps the saddest failure of the OGL, IMHO. With 4e, they sought to address specific problems of 3e -- and, again IMHO, these were real problems that needed addressing -- but they sought to address them in a vaccuum. AFAICT, this was specifically to make a clear break between 4e IP and OGC.
Had 4e been an OGL game, I think we would have seen the same resurgence in interest that we did with 3e. I think WotC's sales would have been greater (and I am not saying that they are not great), that WotC's commitment to the OGL would have made 3pp produce even more OGC, and that this would have given the designers an amazing breadth of materials with which to craft 5e, when it eventually appears.
But I'm just blowing smoke in that last paragraph; I have no way to know.
What I do know is this -- WotC had some really strong early support for the OGL. It was not until later that they began to withdraw that support....right around the time that they were doing "design tests" as it were for 4e. And even then, it was announced that 4e would be an OGL game ("some form" of OGL) for a while, because WotC clearly understood how important the OGL had been.
And still is, to some of us.
All IMHO. YMMV.
RC