States with a small population, such as Alaska, North Dakota, Wyoming, etc. usually have close to the minimum number of electoral votes, which is 3. One for the at large Representative from the state, and two because each state has two Senators.
If you take that as a percentage of electoral votes, 3/538 = .5% roughly, of the total vote. If you take North Dakota's population, 641,481/304 million total population in 2008, then the percentage is much less, .2% of the vote.
So by moving to a national popular vote system (which makes much more sense), small states lose what little voting power they have. Why would any state vote to lessen their own say in the process? Since we need 3/4th of the states to ratify a new Constitutional amendment, that makes one abolishing the Electoral College very difficult to pass.