Question

why Iron cannot displace zinc from its salt solution

Answer 1

Explanation:

Only a metal with higher reactivity rate can displace a metal with lesser reactivity rate.

In this case zinc is more reactive than iron and thus cannot displace zinc

Answer 2

Because zinc is more reactive than iron.                                                        

If we have a look on Metal Reactivity series,we wiil find that iron is lower than zinc in the series.Since the elements are arranged in order of their decreasing reactivity Iron being less reactive than Zinc will not be able to displace it.