The short version is that Toobin says until the Reagan Administration, the Second Amendment was interpreted differently by the courts than it is today.
The Second Amendment reads -- "A well regulated militia being necessary to the security of a free state, the right of the people to keep and bear arms shall not be infringed.” And previously, Toubin writes, the Supreme Court and lower courts gave precedence to the first clause -- the one that refers specifically and only to militias. As such, they'd consistently ruled that it was state militias that had the right to bear arms, but not individuals.
Only after the corporate-owned, right wing NRA got involved pushing their agenda in the 1980s did courts begin re-interpreting the Second Amendment.
And it's worth noting, as we've discussed here and which Toobin brings up, that for those who insist the Second Amendment is inviolate and can't be restricted (never mind the re-interpretation...), we do already have restrictions to the right to bear arms. For instance, you can't own automatic weapons, assault rifles or tanks, among others. Only handguns. It was a convoluted Supreme Court decision crafted by its author, Antonin Scalia.
An interesting side-point that Toobin makes is that conservatives tend to be "original literalists" of the Constitution and scoff at liberals who try to suggest that it is a living document. Yet when it fits their needs, as with the Second Amendment, conservatives are just fine with changing the original interpretation that had been accepted for a couple hundred years.
The article isn't very long. You can read the full piece here.
But if you can't get to it now, it'll still be valid the next time there's a gun massacre.