The motor drive input in the 1-5 volt signal system bees" more noise voltage than the motor drive input in the 4-20 mA signal system.
Follow-up question: what bad effects do you think noise superimposed on the DC signal cable would have on motor speed control?
Challenge question: why do you suppose the 1-5 volt signal system requires a much greater input impedance (1 MΩ) than the 4-20 mA signal system? What might happen to the voltage signal received at the motor drive's input terminals if the input resistance were much less?