It's simply the market at work.This is something of a mantra in US culture, but it's a poor model, because in fact the "market" for research papers may be the least "market"-like thing I know. Papers are funded by grants. There's a handful of agencies that give out significantly-sized grants. The most important of them in the USA - NIH, NSF, DoD - are agencies of the federal government, politically driven and centralized. In the research business as it stands today, the public's "demand" for an esoteric manuscript on, say, the mental health of Drosophila is barely relevant. All that matters is the opinions of your peers, and the opinions of your grant administrator. And these groups don't constitute anything like a "market"; they're more like a family, if perhaps a slightly dysfunctional family. Find a researcher with funding, and you'll find that he or she is almost inevitably well-connected to the research community by a web of patronage and mutual favors.
The peers don't care about open publishing, but apathy works both ways: They won't push for it very hard, but they won't push against it very hard either. What does a researcher really care if the paper comes out in Nature or in PLoS? Sure, there are considerations of culture and fashion - impact factor is not irrelevant to your life as a researcher - but fashion is fickle; old scientists are more adaptable than the average old person and in any case they don't live forever.
Meanwhile, if we gathered the bosses of all the grant administrators in the USA into a room, that room would be the size of an elevator. Convince just a critical mass of agencies - say, the NSF and the NIH - to mandate open publishing and only an act of Congress will prevent the rest of the system from falling in line.
Maybe this isn't right. But, tellingly, the opponents of open publishing have just proposed that act of Congress. A threat to deploy such a blunt instrument doesn't seem like a sign of strength. It seems like a last-ditch attempt to hold back the tide.
(Alas, I agree that the act of Congress could well pass, or even just threaten to pass, and that if it does my whole argument could go into reverse and the status quo will tend to dominate.)