Question

why potential of earth is zero

Answer 1

Potential of earth has been arbitrarily assigned zero.

Due to its large size, we consider it as an infinite source/sink of charges whose potential remains constant with time.

Answer 2

It may be noted that.. engineers like to take earth's potential as zero
physicists like to take infinity as zero potential :D

Answer 3

^^ Point to be noted.. :D

Answer 4

Only changes in potential energy can be measured. Its actual value at a point never enters in any physically measurable quantity.