Towards the end, the author discusses the Breitbart echo chamber of various closely-connected sources of distorted information, and notes that there seems to be nothing very similar on the left. He further notes “There’s nothing in our research that suggests the right is inherently more prone to ideological isolation.”
As a leftist myself, I’d like to suggest two explanations, both of which may have some truth. One is a partisan explanation, the other neutral. First the neutral one: The left in the United States is actually very small. Most of what is called “the left” is actually sort of centre-right by most countries’ standards; it doesn’t hold the ideas that are core to the concept of “the left”, such as class struggle, or indeed many ideas outside the “sphere of debate”. The actual left, the left which is for instance opposed to US militarism, in favour of major changes to the structure of the economy and so on, and therefore discussing mainly ideas which are outside the “sphere of debate”, is very small. It just doesn’t have the critical mass to create a real echo chamber. There are a few sites that do use each other’s stuff, a few writers whose articles will tend to get picked up by leftist outlets, but it isn’t enough to make a serious echo chamber.
The partisan explanation is that the right has more need of an echo chamber. The thing is that what we call “right” and “left” are not really symmetrical, particularly not at the level of mass support for them. Left wing ideology, at its base, is about egalitarianism. Right wing ideology, in the end, is the opposite — it may be specifically about authoritarian leadership, as in fascism, or about (in theory) meritocracy, particularly in situations where the market is emphasized, but it’s always about letting some people and groups rise up, far up. The question about right wing ideology then is, what’s in it for most people? Allowing “entrepreneurs” to become filthy rich is worth it for the average citizen if and only if some variant of “trickle down economics” genuinely operates in the economy. But that does not seem to be factually the case. Thus, support for far right ideology may make sense for aristocrats or the very wealthy or even individuals certain of their exceptional personal qualities (such that they can be confident of great success), but not for the majority. There is in reality nothing in it for them. Thus a mass base for far right ideology necessarily depends on a fair degree of fraud; to get people harmed by inequality to consent to continuing or increasing inequality it is necessary to convince them that they are actually harmed by something else, such as blacks or immigrants or “political correctness” or whatever. When convincing people of falsehoods, broader information is dangerous. Thus an echo chamber is highly desirable.
The left, on the other hand, tends to spend a lot of time looking at more mainstream sources of information and attempting to refute them. The core constituency the left seeks to persuade is the same as the core constituency it seeks to benefit, so it does not need to root its message in fraud, and in turn has no need to fear outside information. There are exceptions — the left has some weird culty groups of dogmatic Communists, who don’t like talking to the rest of the left let alone anyone else — but for the most part the far left has a degree of confidence in its ideas that allows it to test them against the outside in a way that the far right goes out of its way to avoid. There isn’t a genuine symmetry there.