Well, it's his theory, not mine (and indeed I'm not sure if I buy it completely, although his anecdotal data is quite compelling) so I don't know if I'm a good person to answer this.

With that caveat:

1. Part of it certainly is that a lot on the left are isolationist. Some are just in the historical US tradition of isolation, other are that way in reaction to the sheer tawdryness of US foreign adventures from REagan onwards.

2. In terms of dKos diaries, some are just too busy, I know I can barely keep up with the diaries here on ET, so this is a serious effect.

3. Sometimes good people just say little when they have little to say, whilst the emptier cans rattle louder.

4. There is a definite sense that the Repubs have managed to make "cheese eating surrender economists" politcally toxic in the US. Thus, there's a reluctance to be seen supporting multilateralism or European style economics. It's unAmerican! Under such unconscious pressure, sometimes people just go quiet.

I think (4) is a big one. The US is filled with pronouncements about life in Europe. It's hard to see how you could live in that atmosphere and then argue "we need to pull the good things out of the European model and implement them here." It's even harder in the foreign policy arena I suspect.