Why Auroral propagation reflects signals off the rapidly moving, turbulent curtains of ionized particles in the auroral zone, which smears the signal with severe Doppler spreading and a characteristic raspy buzz. CW survives this because the information is carried only by the presence or absence of a tone, so even a hissy, distorted note is still copyable. Voice and digital modes need the fine frequency and phase structure that auroral reflection destroys.
Watch out SSB is the usual DX voice mode and is sometimes used on aurora at VHF, but the distortion makes speech muffled and hard to understand, so it is second best at most; RTTY and other tone-decoding modes fail outright when the tones are smeared.
Aurora = buzzy, warbly signals. Only a mode that just needs on or off survives: CW.