That must be the question conservatives are asking themselves today. After all, the prevailing wisdom (one I signed onto) was that this was a 5-4 decision one way or the other, with Kennedy holding the swing vote. Now, I thought maybe if Justice Kennedy bit, Roberts might come along -- both to keep the decision from being yet another 5-4 ruling and to keep the opinion for himself. But Justice Kennedy voted to strike down the ACA -- indeed, by signing on to a far-right joint dissent for himself, Alito, Scalia, and Thomas, it indicates that it wasn't even a close call for the usually swingy justice. Which means that it was Chief Justice Roberts serving as the middle vote. So what prompted him to (switch his?) vote to uphold President Obama's signature law?
I have two potential explanations floating around right now. The first is legalistic. Chief Justice Roberts has always been a big government conservative. In fact, that's why he was appointed to the Court in the first place -- President Bush wanted someone he could count on to affirm his vast expansions of executive power in the War on Terror. One of the ways I teach my students that judges can exercise some independence from politics is that the political motivations which put them on the Court may not turn out to track the same sets of concerns as their careers progress. The classic example is Justice Frankfurter. Frankfurter was appointed to the Court as a fierce advocate of judicial restraint, which, with the Four Horsemen running roughshod over any and all state and federal economic regulation, was a defining progressive value at the time. And Frankfurter did turn out to be a reliable vote to uphold the New Deal. But as his career continued, the defining controversies for the Supreme Court started to become civil and criminal rights cases. And there, Frankfurter's deference to legislatures led to a far more conservative voting record. I'm not saying Chief Justice Roberts always is going to vote in favor of enhanced government power, only that the particular ideological profile he cuts -- the one that put him on the bench in the first place -- might have made him more sympathetic to the ACA than one might expect from a run-of-the-mill conservative judge.
Second, there are institutionalist concerns that may well have played a role. When he was first appointed Chief Justice, Roberts made clear that he wanted a more unified, less ideologically polarized Court. Whatever else one thinks of his tenure, it is clear that in that respect his has been a colossal failure. The Roberts Court has been bitterly fractured along partisan lines, and more and more (particularly with Citizens United) has been gaining a reputation as an ideologically conservative activist court. It's not that liberals are suddenly going to start singing the praises of the Roberts Court, but had the ACA been struck down, that train would have left the station for good. Basically, Chief Justice Roberts saw in this case that a vote to strike down the ACA was a vote to permanently remake his Court's image into one adjunct to the Republican Party. And he blinked.