Probably because not everywhere on earth has the same easy access that Iceland has. The article mentions this:

> There aren’t gates of Hell just anywhere. A kilometre below ground in Kamchatka is considerably hotter than a kilometre below ground in Kansas. There is also readily accessible geothermal energy in Kenya (where it provides almost fifty per cent of the country’s energy), New Zealand (about twenty per cent), and the Philippines (about fifteen per cent)—all volcanic areas along tectonic rifts. But in less Hadean landscapes the costs and uncertainties of drilling deep in search of sufficient heat have curtailed development.