One of the main reason there is one party in power is because of the American occupation. While the American wanted to get rid of the the most "rightist" factions of the elite for obvious reasons, they quickly change to fight communism (see the book "embracing defeat"). I don't think it is an especially inherent Japanese thing.
People can be very individualistic in Japan as well, it just expresses itself in a different way than more Western cultures. It is very broad generalization, but in the west we like to show ourselves as more individual than we really are, while it tends to be the opposite in Japan. Open conflict avoidance is certainly more common in e.g. corporations in Japan, at least in my experience. But that's a very narrow definition of collectivism.
People can be very individualistic in Japan as well, it just expresses itself in a different way than more Western cultures. It is very broad generalization, but in the west we like to show ourselves as more individual than we really are, while it tends to be the opposite in Japan. Open conflict avoidance is certainly more common in e.g. corporations in Japan, at least in my experience. But that's a very narrow definition of collectivism.