As clear an explanation as I’ve seen:
In Lopez, Morrison, and Raich, the leading Supreme Court decisions on the limits of the Commerce Clause, the Court held that the federal government could regulate minute, barely significant, actions because as a group, those actions have an effect on the economy. But the Court also made clear that Congress could do this only if those actions are economic in nature. It drew this economic/non-economic distinction not from the text of the Constitution (which of course does not use that word), but from the advantages that sprang from drawing the line in that spot—specifically, that drawing the line here allowed Congress to (in Justice O’Connor’s words) regulate more than nothing and less than everything. Drawing the line here was also basically consistent with the Constitution’s language and served the judiciary’s goal of policing the constitutional boundary without intruding into the realm of policy-making. In short, it’s relatively easy for courts to tell when something is not economic in nature, and thus bar Congress from expanding its powers. That’s what’s known as a “judicially manageable standard.”
For the same reasons, it makes a lot of sense, when trying to determine what Congress can do under the Commerce and Necessary and Proper Clauses, to draw the line at the activity/inactivity distinction. While at a certain level, inactivity blends into activity—just as economic and non-economic sometimes overlap—in the vast majority of cases, it’s very easy to judge the difference between acting and not acting. It is empirically verifiable, in most cases, whether a person has voluntarily acted or not. Drawing the line here is relatively easy for courts, and avoids drawing them into complicated policy-making decisions. And, like the economic/non-economic distinction in Lopez and other cases, the activity/inactivity distinction prevents Congress from expanding its power into a general police power.
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