Why do accretion disks around supermassive black holes become extremely hot?

Accretion disks around supermassive black holes become extremely hot because the material within the disk must lose energy and angular momentum to spiral inward toward the central object.12

As matter is pulled toward the black hole by its intense gravitational field, it forms a disk due to its rotational motion. Within this disk, turbulence and viscosity act as forms of friction that dissipate energy and transport angular momentum outward, allowing the material to drift gradually inward. Because the material must shed this energy to accrete onto the black hole, the process generates significant heat.12

This conversion of gravitational potential energy into radiation is highly efficient, often heating the disk to temperatures ranging from a few thousand to several million kelvins. Consequently, these disks emit energy across a broad spectrum, ranging from infrared to X-ray wavelengths.12