Because, first, few know. The courts have ignored the Ninth and Tenth for so long, people simply do not know how US federalism is supposed to work. The issue does not occur to them. For the few for whom it does occur, the fact that Civil War successionsits and later apologists both couched the motivation for the war in the language of States’ Rights to disguise the preservation of slavery motive, the idea has been politically tainted.
Throw on that it is more complicated to explain that straightforward federal rule — modern journalists are not a knowledgable or researching lot — and yes, it doesn’t get mentioned.