Why is geothermal energy unlikely to become a major energy source?
Right now at least, deep geothermal is only economic when you have a town/city located close to a geothermal hotspot of some kind.
Deep geothermal energy depends on a location where the geothermal gradient is quite high and close to the surface. Places like Iceland for example are close to underground hot magma chambers that heat hot water and are quite close to the surface.
Most places in North America are in this sense, not that favourable for geothermal except in the western provinces and states. This is where plate tectonics is subducting great slabs of rock and the geothermal gradient can locally be quite favourable.
The other problem is that the best spots for geothermal are not necessarily where cities and towns are located. See the example from Western Canada.
However, a new kind of shallow geothermal called ground source heat pumps only require shallow sources of heat and can be economic in a number of locations. As this technology becomes more efficient, they may have much greater utilization. See 2nd pic.