A few reasons why the Japanese government does not (but should) order the evacuation of Fukushima City (pop. 260,000)

It's beyond comprehension why the Japanese government has not ordered the evacuation of Fukushima City. Below I list some possible reasons that explain, but by no means excuse, this failure. These are sentimental and financial considerations that are getting in the way of rational thinking about the necessary measures that need to be taken.

1. Other cities in Japan have not been exactly keen to welcome evacuees from Fukushima. Despite Japan's reputation for being a unified, group-oriented culture, most people maintain fairly small horizons and their loyalties are to smaller communities. Many farming communities and small towns all over Japan have had a declining population, so one could easily imagine a resettlement program to put people from Fukushima in these areas, but the evacuees are not emotionally ready to resettle, and few towns are ready to welcome them. The government implicitly knows that evacuees would face discrimination (i.e. in finding a spouse) and serious problems of adjustment to a new location.
2. Many areas of Fukushima outside the limited evacuation zone have levels of contamination comparable to places near Chernobyl that were evacuated quickly by government order. People around the world obviously wonder why a developed, wealthy nation cannot do the same for its own people. But the wealth and high standard of living is precisely the problem. It has allowed people to deceive themselves that the tragedy of relocation under government order could be something that only happens to the miserable and downtrodden in other places.
3. The evacuation of Chernobyl was also easier to implement because the population had already been dispossessed sixty years earlier by the communist revolution. Many of the residents of the area had come from other parts of the Soviet Union in recent times. In contrast, most people in Fukushima have ties to the land going back centuries. They have emotional bonds to rituals such as visiting ancestors graves, not to mention all the deep bonds to their communities.
4. The biggest difference is that, in the case of Fukushima, we are talking about a capitalist, democratic society. In order for the government to order an evacuation, it would also have to order the nation's banks to write off every outstanding loan of the businesses and individuals who would leave. Such an evacuation is unprecedented in a capitalist democracy. The Soviet Union was a top-down dictatorship that could simply order people to abandon land that they didn't own anyway. In Japan, no single element of the corporate world or government has the power to order the evacuation of a city the size of Fukushima and adequately support the evacuees.
5. Anyone with an above-average social status or income would be opposed to seeing the city evacuated. Even people employed at low incomes would still prefer to be gainfully employed. Even if they could claim benefits as an evacuee, they would face an undesirable loss of status and satisfaction in their employment. The wealthier individuals could afford to send their own children to safer locations, without government support.
6. Some might say we should just evacuate the children and mothers for a while, but this would equal the eventual decline of the city. Everyone would know they wouldn't be coming back soon, and everyone would know: no children, no city. Fukushima City would just begin a rapid economic decline shortly after the children left.
7. Japan used to be called "the country that can't say no" but a recent book by an American diplomat calls it "the country that can't decide." In Japanese culture, hard decisions are avoided, and unpleasant truths are never stated explicitly. When people were evacuated from the villages within 20 kilometers of the nuclear plant, they were told it would be temporary, but the implicit message was known by anyone who had learned even a little of how the area compared with the Chernobyl exclusion zone. It was six months later when a government official was finally able to state, as frankly as is possible in Japanese culture, that no one would be able to come back "for a long time." If Fukushima City needs to be abandoned, it appears that it will take "a long time" for Japanese society to face up to the fact.

Although there are some unique features about the situation in Japan, a nuclear disaster might play out the same way in other countries. It is likely that economic considerations would win out over a cautious approach to children's health. The financial system would not agree to write off all the mortgages it held in the evacuation zone just for the uncertain gamble that the evacuation is necessary to avoid a possible health catastrophe in a few years time. This is why nuclear power is too dangerous to allow to continue. No other industry has the potential for such widespread, lasting destruction.