There's a number of reasons. One, a lack of knowledge about nuclear accidents. It's a low freq, high severity risk but the freq is so low there's almost no data to go on about. Insurance companies only work with LOBs they know about. Two, the severity in a worst- case scenario would be so high that it would be the cat to end all cats. This would cause the insurer to place restrictions such as a time- limit on loss development (since people would get sick and sue 20 years later) and the losses outside those restrictions would probably be high enough that it wouldn't even make sense for the plant to buy the insurance. In short, insurance companies don't like insuring a bunch of risks with infintesimal freqs but infinitely high severities, especially when dealing with technologies they know nothing about.