Not surprising at all, particularly when then-Prime Minister Noda said back in March 2012 that no one in particular was at fault, we were all at fault.
When everyone is said to be responsible, no one is responsible.
All Japanese papers "leaked" this story on August 8, 2013. Some leak. A very detailed one at that, too.
Here's one from Asahi Shinbun (8/8/2013; part):
All people concerned, including former chairman of TEPCO and former Prime Minister Kan, won't be prosecuted
It was revealed on August 8 that the Public Prosecutors Office is coordinating [among themselves] so as not to prosecute any one of those who have been charged for professional negligence resulting in deaths and injuries over the Fukushima I Nuclear Power Plant accident. They include TEPCO's top management and government officials. The prosecutors are expected to announce this non-institution of prosecution by the end of August.
Total 15,000 people, mostly people near the plant and affected by the nuclear accident, have been filing charges intermittently since the accident in March 2011 that they suffered injuries, such as hospital patients having died during the evacuation right after the accident, and residents exposed to radiation. The charges were against former Prime Minister Kan, TEPCO's former chairman Tsunehisa Katsumata, former president Masataka Shimizu, Nuclear Safety Commission's chairman Haruki Madarame, former Chief Cabinet Secretary Yukio Edano, former Minister of Economy, Trade and Industry Banri Kaieda, and dozens of others. The charges were received by the prosecutors in August last year. The Public Prosecutors Office has asked for help from prosecutors in the Tokyo District Prosecutors Office and Fukushima District Prosecutors Office, and has been questioning.
...As the result, [the Public Prosecutors Office] has come to the conclusion that an earthquake and tsunami on such a large scale cannot be said to have been predicted by experts before it actually took place. As to the cause of hydrogen explosions in the reactor buildings, they have dismissed the charges that the delay in vent to lower the pressure inside the Containment Vessels led to the hydrogen explosions; instead, the prosecutors decided that the delay was caused by high radiation levels and power outage. The Public Prosecution Office is expected to conclude that it will be difficult to establish criminal liability of former Prime Minister Kan and TEPCO management.
Never mind that an earthquake and tsunami of that magnitude have actually been predicted by experts. Never mind that the government under Naoto Kan hid the data of radiation levels "so as not to panic the populace", when it was them who panicked. It was those bureaucrats at NISA who had the more accurate simulation of radiation dispersion and were about to set the more precise evacuation areas, but completely dropped that effort as soon as they learned that Kan, Edano, and Kaieda came up with those childish concentric circles.
Never mind that it was the government who did nothing to stop farmers in Fukushima from tilling the contaminated land and plant crops right after the March 11, 2011 accident.
A UN committee member spoke of Japanese judicial system as "medieval" where the prosecution almost always wins, near 100%. The Japanese prosecution hardly ever even thinks of indicting the national government officials and powerful corporate elites, and there is no grand jury system in Japan.