Question

Why is it necessary to clean the nichrome wire in between metal ions?

Answer 1

In doing a flame test the ions of the previous metal will adhere to the nichrome wire, causing both spectrums to be present

Explanation:

Each element has a unique spectrum due its electron configuration.
The element can be identified by the spectral lines in the flame test.
However if the nichrome wire is not cleaned between the testing of subsequent metal ions there is a problem. The spectral lines of both (or more) metal ions will be present in the spectrum. This will make identifying the second or new ion very difficult.

Cleaning the nichrome wire will limit the spectral lines seen to one element making the identification easier.