The operating pressures here are actually much higher than steam. Supercritical fluids are extremely interesting because they have some helpful characteristics

The operating pressures here are actually much higher than steam. Supercritical fluids are extremely interesting because they have some helpful characteristics of liquids, like density, and some helpful characteristics of gasses, like heat expansion. For this reason, supercritical fluids in general are the subject of a lot of heat engine research. I believe supercritical CO2 comes to the table because it becomes supercritical at a low enough temperature and pressure that it won't take major breakthroughs in materials science in order to build a heat engine with it.