Why do fusion reactions require much higher temperatures than fission reactions?

The fundamental reason fusion reactions require significantly higher temperatures than fission reactions is the presence of the Coulomb barrier.1

In nuclear fusion, the reactants consist of light, positively charged nuclei. Because like charges repel each other, these nuclei experience a strong electrostatic repulsive force that prevents them from getting close enough to fuse. To overcome this barrier, the particles must possess extremely high kinetic energy, which is achieved by heating the fuel to temperatures of millions of degrees, creating a plasma state.12

In contrast, nuclear fission typically relies on neutrons to induce the splitting of heavy nuclei like uranium or plutonium. Because neutrons carry no electric charge, they are not subject to the Coulomb barrier and can enter a nucleus even at room temperature. Consequently, fission does not require the extreme thermal energy necessary to force charged particles together, which is the primary hurdle in achieving controlled fusion.13