In the 2000 presidential election Arizona had 8 electoral votes. What best explains the change shown on this map

by 2012, a higher percentage of the countrys population lived in Arizona

You didn't show a map, but I can explain how electoral college votes are determined. Arizona's number of electoral votes has been growing because its state population has been growing at a rate faster than other states, and some states have seen shrinking of their population.

Here's what the National Archives says concerning how Electoral College delegates are assigned: Electoral votes are allocated among the states based on the Census. Every state is allocated a number of votes equal to the number of senators and representatives in its U. S. Congressional delegation-two votes for its senators in the U. S. Senate plus a number of votes equal to the number of its members in the U. S. House of Representatives.

So the number of electoral votes each state gets (of the 538 total electoral votes) is recalculated every ten years, based on the most recent US Census data.

In the 2000 presidential election, Arizona had 8 electoral votes, as it had also in the 1992 and 1996 elections. That number was based on the 1990 Census figures for population. In the elections previous to that, in 1984 and 1988, Arizona had 7 electoral votes, based on population numbers from the 1980 Census. In the elections of 2004 and 2008, Arizona had 10 electoral votes based on 2000 Census data. In 2012 and 2016, Arizona had 11 electoral votes based on the 2010 Census. So you can see that Arizona's relative share of the national population has continued to grow and affect its share of Electoral College votes.