Why Does Webb’s MIRI Need a Cryocooler if the Telescope Already Has a Sunshield?

I understand that the James Webb Space Telescope’s sunshield blocks heat from the Sun, Earth, and Moon, allowing the telescope’s cold side to cool passively. What I cannot quite connect is why the Mid-Infrared Instrument (MIRI) still needs a cryocooler to reach roughly 7 kelvin.

Is the extra cooling mainly needed to reduce thermal radiation from the instrument itself, electrical noise in its detectors, or both? Why is passive cooling sufficient for Webb’s near-infrared instruments but not for MIRI?

An explanation connecting infrared detector cooling to the wavelengths being measured would help more than a description of the cryocooler’s mechanical parts.