Some people just don't like the Electoral College.
True, it's not "one man, one vote". It was never designed to be. This nation was founded as a republic, not a democracy. The Congress isn't "one man, one vote", either, since each of the states has the same number of senators. The electoral college, like the twin-house Congress, was a compromise necessary to create our nation. It's served us well for over 230 years.
It's happened before, but in 2000, George Bush won in electoral votes but not in the popular vote. Democrats have never forgiven President Bush for winning according to the rules, and neither have they forgiven the Founding Fathers for creating the system that allowed President Bush to win the election.
So they want to change the rules by creating the Interstate Voting Compact.
The Constitution gives to the states the authority to choose their own presidential electors, and with this compact states would agree to give all their electors to the winner of the national popular vote. It's an attempt to get around the Constitution, and this post spells out some of the legal pitfalls of the idea.
I have a better idea, one easier to implement and less fraught with Constitutional peril. Why not just have the state allocate electors in proportion to the vote count in that state? In other words, if Candidate X gets 43% of the vote, Candidate Y gets 38% of the vote, and Candidate Z gets 19% of the vote, just designate 43% of the state's electors to vote for X, etc. That's much more "one man, one vote"-like than what California and many states currently do, which is to allocate all of the state's electors to whoever wins the popular vote in the state.