Because unless you sit on top of a volcano, amount of renewable geothermal energy is minuscule. In most places on Earth it's somewhere around 40 mW/m2 (i.e. accounting for conversion losses you need to capture heat from ~500 m2 to renewably power one LED light bulb!). In other words, in most places geothermal plant acts more like a limited battery powered by hot rock, so unless drilling is extremely cheap, it does not make economic sense compared to other energy sources.