I believe building geothermal plants are kind of gambling, nobody knows if the plant could produce enough electricty which is worth for the investment. Likelihood of success may be higher in some volcanic island such as Iceland, but Japan is different. This increases the investment when building the plant. That was one reason I heared when I was a college student.

Also there must be a financial reason. Japan already has other stable electrical sorces such as gas, nuclear and waterfall. If one of them will be replaced, geothermal plants can be a candidate and the likelihood of its acceptance will increase as the technology around the geothermal plant glows.