Why Bandwidth of a digital signal depends on its symbol rate and, for FSK modes, the tone spacing. FT8 uses 8-tone FSK with a symbol rate of about 6.25 baud and tones spaced 6.25 Hz apart, so the whole signal fits in roughly 50 Hz. That is far narrower than the other modes listed, which is why FT8 works at signal-to-noise ratios well below what the ear can hear.
Watch out The 170 Hz shift, 45-baud RTTY signal is the usual runner-up, but the shift alone is 170 Hz and the occupied bandwidth is around 250 Hz. MFSK16 runs about 316 Hz, and PACTOR IV spreads across roughly 2.4 kHz to get its high data rate.
FT8 = slow tones, tiny footprint: about 50 Hz. More speed or more shift means more bandwidth.