Under the U.S. Constitution, the states have exclusive and plenary (complete) power to allocate their electoral votes, and may change their state laws concerning the awarding of their electoral votes at any time. Under the National Popular Vote bill, all of the state's electoral votes would be awarded to the presidential candidate who receives the most popular votes in all 50 states and the District of Columbia. The bill would take effect only when enacted, in identical form, by states possessing a majority of the electoral votes—that is, enough electoral votes to elect a President (270 of 538).
The NPV plan requires a winner to secure only a plurality rather than a majority of the popular vote. While it is true that a majority of the popular vote is not required now, the current system requires that, at minimum, pluralities be achieved in at least a dozen states holding distinct elections.
Under NPV, the necessary plurality could be confined to a few states, or a single region of the country. Multiple regional or even favorite-son candidacies would be encouraged, and each new candidacy would increase the likelihood of one of them receiving a majority of the electoral votes (courtesy of the NPV compact) while capturing a very low percentage of the overall vote. If there were four major candidates in the race, victory could be achieved with just over 25% of the popular vote.
The bottom line, to borrow from former Defense Secretary Donald Rumsfeld, is that there are too many "known unknowns." Is it worth the risk to remedy an ill-defined problem that historically occurs once in an average lifetime?