GR was focused on True20 before 4e was announced, and Paizo made the Pathfinder decision based on the fact that they needed to make a decision about their direction and hadn't seen the 4e rules yet. According to the people involved, the GSL didn't factor into those decisions, since there were little to no details on it at the time. James Jacobs even directly stated that the GSL wasn't the motivation for Pathfinder.
Read what I said next time. "Complete focus."
Lots of companies have their own system and also publish stuff for other systems. Lots of 3pps used to do that with d20. If there was a 4e OGL, I suspect Paizo, GR, etc. would publish *something* for them. Heck, they're all still publishing 3.5e/d20/OGL stuff along with their new lines.
Green Ronin has done Freeport Companions for True20, d20, Savage Worlds, and Castles & Crusades. Would they really *not* do one for 4e if it was a reasonable choice? But except for one character folio, they're not touching it. Reason? GSL.
Paizo's got an aggressive publishing schedule of many APs, supplements, and standalone adventures (still in 3.5, I might add). Why zero 4e? Why wouldn't they publish some adventures at least? Sure, Pathfinder's their direction, but WotC has had to work at being excluded totally. Reason? GSL.
Mongoose. They support a wide variety of lines. People that talk about "oh, 3pps aren't publishing much these days" aren't in touch with reality, as Mongoose is cranking out more than a dozen products per month. (Even Paizo is matching WotC's RPG output; they are far exceeding it.) And exactly one of those has been 4e (Wraith Recon).
If your contention is that the GSL has nothing to do with this state of affairs... Then what does exactly? (Real reasons, not "oh all the 3pps are out of business anyway" or "companies other than WotC aren't publishing much any more".)