From Chris Pramas. To quote in part:
"It became clear during this period that there was a faction with WotC that wanted to close the door to third party publishing all together. There were also advocates, most notably Scott and Linae, but it seemed they were in a constant battle to make anything happen at all. The resultant license, the GSL, looked like an attempted compromise between the factions within WotC that probably pleased no one. It certainly pleased few of the established third party publishers. So within two months of the release of the original GSL, a revision was announce to address some of these concerns. It took over six months for that to happen, and while the revision has some improvements the core of it is very similar indeed to the original.
Now while this was all going on, Green Ronin was by no means standing still. We had existing lines like M&M and True20 to support, a new game line in A Song of Ice and Fire to launch, and new deals to negotiate. The company had begun diversifying away from d20 material many years earlier so it was really just a case of continuing that momentum. While d20 was good to us and we published some great books in that era, we ultimately got to a place where we controlled all of our own lines and were beholden to no one.
So when the GSL revision came out, I had to ask myself if I wanted GR to get pulled into WotC's orbit, even a little bit. The answer had to be no. I don't ever want to have to wonder again what a new edition of D&D means to my business. I don't want to worry about whether 5E or 6E is going to be open to third party publishers. I don't want to live with the spectre of the wrong person becoming an exec at WotC and wrecking my business with the stroke of a pen. It's just not worth it, particularly for the level of sales we'd be likely to see doing 4E support. (The best anyone has been able to say about sales of third party 4E stuff is that it's better than late era 3.5 sales, which is like saying that Friday the 13th Part 13 sold more tickets than Friday the 13th Part 12.)"
This is going to be a bigger knock to some folks than the expected Paizo announcement, but I see his reasoning. On the plus side for 4th-ers, Fat Dragon Games and a couple of small-press companies are in, according to this thread at ENWorld. Clark Peterson of Necromancer appears to be pushing pretty hard in that thread, but so far the GSL continues to be a mixed bag in terms of publisher reaction, however improved the license may be.