Why Repeaters use angle modulation, FM or its close cousin PM, because the signal amplitude stays constant. That lets a receiver's limiter strip off noise and amplitude variations, and the FM capture effect makes the strongest signal dominate so you hear clean audio instead of two stations mixing. Constant-amplitude signals also allow simple, efficient transmitter amplifiers, and the wider VHF/UHF bands have room for FM's roughly 10 to 15 kHz bandwidth. PM sounds the same through an FM receiver, which is why the pool pairs them.
Watch out SSB is the right pick for weak-signal work on the VHF/UHF CW and SSB segments, but it needs precise frequency tracking and has no capture effect, so it is not used for FM voice repeaters.
Repeaters and packet = FM; weak-signal DX = SSB. Constant amplitude means quiet, captured audio.