why must use 4-20mA?

There is a long tradition dating from the early pneumatic instruments for a control range from 25% to 100%. If you want to know why, look at the pressure vs travel curve for a flapper-nozzle. The early electronic instruments used the same force-balance approach (many with a 10-50 mA range - easier to get a higher force with the higher current) which was eventually translated over into the pure electronic format.

4 mA is enough to supply the standing current needed for loop power and not enough to cause excessive power loss. Why not 5-21 mA? They had to decide on something. My pick would have been 5 -25 mA as it is slightly easier to convert a current into a % signal with the 20 mA span