Its not really supposed to have a popular mandate. The role of the courts is to establish primacy of law. If two legal standards conflict, individuals need to know which one has precedence. Otherwise, you end up with situations in which everything you do (or don't do) is functionally illegal.

When a Congress accumulates legislation over time that results in contradictory legal doctrine, it isn't the court's job to appease the public. Its the court's job to untangle the mess. Congress is then given the job of re-legislating to establish a legal code that has popular approval.

There are a lot of things wrong with the modern courts. But they are, at their heart, simply filling a bureaucratic role. They aren't supposed to be setting policy. They're supposed to be shaping policy such that it is a thing people can actually abide by.