Their highly speculative "plans" on when the problem will be "solved" talk of some 30 years or more.
The cause of the loss of workers are of course extreme radiation and very bad working conditions. Radiation exposure is clearly a major factor and is explicitly admitted by the giant energy company.
Robots and other machines are also susceptible to radiation damage if the levels are as high as they actually are, so they can only be used up to a point. In Chernobyl they eventually had to use human workers in very short shifts because robots just broke down because of the damage caused by radiation to their circuits and mechanisms.
Many workers are there just because they owe too much money to the Yakuza and have no realistic alternative if they want their families to survive. Others are long-term unemployed and other desperados.
They also admitted that the building of reactor no. 4 is very badly damaged and will not withstand the next serious earthquake.