The basic facts are that the pedestrians should feel they need to get out of the way, but aren't feeling any such thing. It's debatable as to whether it even occurs to them intellectually. Bystander effect. "Even if I move, the rest of these jackasses won't, so why bother?" Also, I have little doubt their 'education' system is basically the same Prussian BS as ours. Which means nobody tells them basic basic stuff like the fact that sirens have right of way. Even the ones that realize the ambulance wants them to move are likely learning it on the spot from the van's body language, so to speak, and subsequently wondering why the ambulance is being a jackass.

I say we need more evidence. One video clip is just too short to make much of a judgement, there could have been some factor we didn't see. The Japanese are courteous people who demonstrated altruism in the recent earthquake. So, my two cents: We aren't seeing the whole picture here.

Indifference of crowds. In that situation it's very easy to rationalize that your own decision (stop vs. keep walking) is insignificant since a hundred other people are still blocking the way no matter what you do. You can route the responsibility for the situation to the collective shoulders of a mass of people. There might also be a theory on 'decision inertia' or such that it's always more difficult to change your action than keep doing what you do. You probably remember not wanting to go somewhere as a kid and then not wanting to leave once you got there. The people in the film are already moving, and stopping would require a direct decision to alter their planned course of action. Anonymity in public is a huge factor for people being passive in different situations. In emergencies with a lot of bystanders, you shouldn't make a general call for help "Someone call the police!". Instead, you should make precise requests for individual people. "You, call the police. You, go stand in that corner and warn incoming traffic". If it were just a small but steady trickle of pedestrians, they would stop and leave a gap for the ambulances, no matter if it was in Shanghai, Nairobi, or Islamabad. Or maybe that's just my euro-centric wishful thinking. So yes, 'high population density sucks'.

Looking at the translated youtube comments it appears that other Japanese are shocked by the behavior of these Tokyo pedestrians, so I'm going with population density as the prime cause.

hbd3 cult1 sounds good to me.

cult4 hypothesis: People have been desensitized to the urgency of sirens through constant exposure and seeing examples of them being used for the convenience of the operators rather than for emergencies. In NYC I've heard in response to fire engine sirens "looks like the diner is on fire again" - in other words there's no fire but the firefighters don't want to wait in traffic. No idea if this is the perception in Tokyo.