Question

why can the pace or a footstep not be used as a standard unit of length​

Answer 1

The measurement of pace or footstep varies from one person to another person.

so, It can't be taken as a standard unit of length.

...Hope you got it...

Answer 2

Pace or a footsteps can't be used as standard unit of length because everyone's footsteps are not equal.There will be controversy.