Why A digital signal's occupied bandwidth grows in proportion to how fast the symbols change. Each symbol transition is a change in the waveform, and faster changes create sidebands spaced further from the carrier, so doubling the symbol rate roughly doubles the necessary bandwidth. That is why slow modes like PSK31 (about 31.25 symbols per second) fit in under 100 Hz, while fast packet or high-speed data modes need many kilohertz.
Watch out The choice saying bandwidth is half the symbol rate inverts the Nyquist idea: the minimum bandwidth needed is on the order of the symbol rate, not half of it, and real signals with filtering and keying sidebands take somewhat more.
Faster keying, fatter signal. Symbol rate up, bandwidth up.