A popular vote scheme would be vulnerable to fraud. Currently we have a decennial census to determine states' populations and assign electoral votes accordingly (along with senators). Since the Constitution recognizes the States power to control the election process a state wishing to commit fraud would simply create millions of multiple voter registrations and stuff the ballot boxes to overwhelm the national numbers. Under the current system states can commit all the fraud they can get away with but the damage is limited by the set number of electoral votes allocated to each state.
If you thought the Florida recount process was a circus, imagine if every state had to do a recount to make every vote count. Imagine how many additional ballot boxes would be uncovered.
The main criterion for how to evaluate the merits of any voting system is how it deals with concerted efforts by insiders to defraud the system—because that’s what any voting system has to deal with as a practical matter. That is one of the main reasons why the electoral college exists. Under the current scheme, it doesn’t matter if LA or NYC report 125% voter turnout—local fraud cannot sway a national election. Switch to a national popular vote and that’s exactly what to expect.