The answer lies in another part of Article IIthe part that received some important attention in Bush v. Gore, the Supreme Court decision that Democrats love to hate. Famously, the 2000 case between Republican nominee George W. Bush and Democratic nominee Al Gore effectively handed the election to Bush when it ended the Florida recount.
Article II provides that the state legislature may direct the manner for choosing presidential electors. In Bush v. Gore, the Court stated that this Article II power given to state legislatures was plenary, meaning that the states have a broad power when it comes to presidential elections. Indeed, the Court wrote that even though state legislators have given each states voters the right to vote for presidential electors, at any time a state legislature can take back the power to appoint electors. In other words, if the California or Texas state legislature wanted to directly choose the states presidential electors in 2020, the state could do so. As Dean Vik Amar notes, the Constitution does not necessarily include a right of Americans to vote for president at all (and American citizens in U.S. territories do not have this right).
https://www.politico.com/magazine/story/2017/03/donald-trump-tax-returns-release-214950
It's too bad that we're down to forcing a financially secretive candidate to simply volunteer to do what has been considered basic ethical transparency in presidential candidates since the seventies.
But 2016 showed we can't expect that.