Julian Sanchez has been writing about the right's tendency toward "epistemic closure" -- an intellectual world in which the only trustworthy sources of information are those within your movement. Sanchez is libertarian-ish, but clearly argues that this is primarily a right-wing phenomenon:
I’m often tempted to pluck some instances from the left just to show how very fair-minded and above the fray I am. (For instance, I find myself increasingly sympathetic to complaints about the coverage of the Tea Parties: Obviously there are both subtle and not-so-subtle bigots in the pack, but I doubt they’re representative, and it’s a huge leap to the dismissive suggestion that the phenomenon is nothing but a manifestation of racial anxiety.) Yet I can’t pretend that, on net, I really see an equivalence at present: As of 2010, the right really does seem to be substantially further down the rabbit hole.
Perhaps some of that perception can be put down to the fact that I mostly write about the issues where I’m prone to agree with progressives—so I’m more conscious of it when Fox spins fantasies about the Patriot Act than when MSNBC spins on economics or health care—but I don’t think that’s the whole of it, since I feel like I see the same tendencies even on issues where I’m closer to the conservative position. So suppose it’s true that there’s a real asymmetry here—the obvious question, if we’re going to sideline the cheap partisan explanation that conservatism intrinsically appeals to the stupid or closed minded, is why this should be true now.
To answer his question of why, Sanchez posits that the right may be reacting to a new media world in which they're suddenly exposed to a nationalized media market, in which they're suddenly exposed to different thinking people from across the country and even the globe, and need to shut out uncomfortable heterodoxy. I find this explanation unsatisfying. The right has been creating an alternative media network for more than thirty years, deriding all mainstream news sources as hopelessly biased and encouraging fellow believers to trust only information produced by sources within the movement. In any case, I don't see the average conservative as being appreciably more bombarded with viewpoints from the coasts then they were during the era when everybody watched Walter Cronkite.
If technology is playing a role here, it's probably allowing for a more totalistic alternative information cocoon. Conservatives always had access to conservative opinion, but the rise of cable and the internet allowed them to create news sources that totally replaced, rather than merely supplemented, the mainstream media.
Of course this answers the question of why now?, but it does not answer the question of why conservatives and not liberals. Matthew Yglesias suggests that the progressive movement is too diverse to be controlled:
The left is simply less monolithic. It seems to me that if you look at the discourse among “green” types, you see groupthink there. And if you look at labor types, there’s another groupthink there. And there’s an immigrants’ rights groupthink and there’s feminist groupthink and all kinds of groupthink all around. But these points of view come into contact with one another and only partially overlap. At times they conflict. The progressive infrastructure contains people and institutions who are robustly on both sides of important questions like trade policy or K-12 education. Business groups are very involved with most Democratic Party politicians and with many progressives organizations (we have a “Business Alliance” at CAP). I think it would actually be beyond the intellectual powers of any one person to work all the sacred cows of all the different factions of the movement into a seamless and coherent whole.
The right just isn’t like that. It’s less demographically diverse, less diverse in its financial base, and less ideologically diverse.
This answer also strikes me as unsatisfactory. You can easily find examples of left-wing communities that are economically or ideologically diverse yet have epistemic closure -- The New York Times can be ignored because it's the corporate-controlled "New York Crimes." It's just that these communities are small and generally reside outside the two-party structure.
I think the answer is that liberalism is not really an ideology in anything like the sense that conservatism is. Conservatism is an ideology organized around the belief that big government inherently destroys freedom. Contemporary liberalism is the ideology of people who don't share that conviction, though it lacks any strong a priori beliefs to hold it together. I wrote about this in a 2005 essay for TNR's 90th anniversary issue.
Liberals are not ideologically pro-government in anything like the sense that conservatives are ideologically anti-government -- conservatives view shrinking government as an end in and of itself, while liberals would view expanding government a success only to the extent that doing so furthers some other real-world benefit. I think it's the fundamental distinction between the two parties, and it explains all kinds of asymmetrical behavior -- a loose coalition versus a coherent ideological movement.
Now, I realize that I'm only discussing economics, and while this is the central front for two-party competition, it's not the only front. I don't think I have the only answer here to Sanchez's question. (Indeed, on social issues and foreign policy, I think there's more symmetry than asymmetry between the two parties.) I do think the central role of economics in the two party competition does play in important role in organizing the contrasting epistemological styles of liberalism and conservatism -- that is, economic conservatism plays a dominant role is shaping the epistemological style of the conservative movement as a whole, and likewise for economic liberalism.
Sanchez admirably dismisses "the cheap partisan explanation that conservatism intrinsically appeals to the stupid or closed minded." That's certainly an explanation we should treat with caution. But should it be dismissed out of hand? Open-mindedness to rational inquiry is a political style historically linked with liberalism, and it's usually (though not always) found more often in liberal parties than in conservative or Marxist ones. Certainly, when we consider other countries, we frequently assume that one party is more nationalistic, populist, reactionary, racialist, fronting for powerful economic interests, and so on, and often we associate those parties with simplistic or closed-minded approaches to politics. Likewise other parties are associated with technocracy, internationalism, and general willingness to impose policy reforms in response to objective needs. We don't assume that there's some universal law requiring the spirit of open-minded inquiry to be equally divided between the two major parties in any democracy. Nor should we assume that such a law should apply to the United States but not elsewhere.