Why not go with the nationwide popular vote?
Federalism.
We can still be Federalist with a popular vote for President
If an amendment is passed, sure.
The U.S. Constitution says "Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors . . ." The U.S. Supreme Court has repeatedly characterized the authority of the state legislatures over the manner of awarding their electoral votes as "plenary" and "exclusive."
The normal way of changing the method of electing the President is not a federal constitutional amendment, but changes in state law.
Historically, major changes in the method of electing the President have come about by state legislative action. For example, the people had no vote for President in most states in the nation's first election in 1789. However, now, as a result of changes in the state laws governing the appointment of presidential electors, the people have the right to vote for presidential electors in 100% of the states.
In 1789, only 3 states used the winner-take-all method (awarding all of a state's electoral vote to the candidate who gets the most votes in the state). However, as a result of changes in state laws, the winner-take-all method is now currently used by 48 of the 50 states.
In 1789, it was necessary to own a substantial amount of property in order to vote; however, as a result of changes in state laws, there are now no property requirements for voting in any state.
In other words, neither of the two most important features of the current system of electing the President (namely, that the voters may vote and the winner-take-all method) are in the U.S. Constitution. Neither was the choice of the Founders when they went back to their states to organize the nation's first presidential election.
The normal process of effecting change in the method of electing the President is specified in the U.S. Constitution, namely action by the state legislatures. This is how the current system was created, and this is the built-in method that the Constitution provides for making changes. The abnormal process is to go outside the Constitution, and amend it
The National Popular Vote bill would replace state winner-take-all laws that award all of a state’s electoral votes to the candidate who get the most popular votes in each separate state (not mentioned in the U.S. Constitution, but later enacted by 48 states), in the enacting states.
The bill retains the constitutionally mandated Electoral College and state control of elections, and uses the built-in method that the Constitution provides for states to make changes. It ensures that every voter is equal, every voter will matter, in every state, in every presidential election, and the candidate with the most votes wins, as in virtually every other election in the country.
Under National Popular Vote, every voter, everywhere, would be politically relevant and equal in every presidential election. Every vote would matter in the state counts and national count.
The National Popular Vote bill would take effect when enacted by states possessing a majority of the electoral votes—270 of 538.
All of the presidential electors from the enacting states will be supporters of the presidential candidate receiving the most popular votes in all 50 states (and DC)—thereby guaranteeing that candidate with an Electoral College majority.