The NPV is an end run around the compromise between less populous and more populous states that is fundamental to our Constitutional republic. If it ever passed and electors tried to vote differently from how their own state voted, it would throw the country into chaos. Voters in such states would sue and the state Supreme Courts or the US Supreme Court would decide the election.
The big problem is that an NPV winner who only wins because of the NPV would be regarded as illegitimate by a majority of the people in states with over 270 electoral votes. What if those states decide to recognize the true Electoral College winner as President?What if the US Supreme Court throws out the NPV winner, but half the NPV states refuse to accept the ruling? There are just too many disaster scenarios with the NPV. It’s much too risky.
The only way we can move to a popular vote approach that achieves legitimacy and avoids potential chaos is to pass a Constitutional amendment. That would require majorities in many states with small populations to cede the advantages they have under the current system. What could be offered to them in order to arrive at a new and different compromise?