Technological conveniences

The mid-21st Century has many technological conveniences. My favorite, which would be the envy of denizens of the early 21st Century and before, is the ability of cars to stop next to a parking place and "step over into the place". Eliminating jockeying a car, back and forth, into a parking place is such an obvious step that you might wonder why having such a feature took so long. Apparently, there was too much complication for such a feature to be economically viable a rather short distance back in time. Another major step forward was eliminating gasoline as the energy source. A variant of hydrogen fuel cells essentially made the oil in the ground in the Middle East and Venezuela close to worthless. For relatively clean power, not without complications, water is turned into hydrogen and oxygen, using electricity from newer generation nuclear power plants. Why was there such a delay in using a nuclear-heated thermocouple to generate electricity?