Hidden variable theories are not an attempt to ground the lawfulness of quantum mechanics. The Schrodinger equation isn't reduced to something deeper in Bohmian mechanics. It appears as a basic unexplained law in the theory, just as it does in orthodox interpretations of QM. The motivation behind hidden variable theories is to repair purported conceptual defects in standard presentations of QM, not to account for the existence of the laws of QM.

I do think my claim is wrong, though. People do ask what grounds quantum field theory. In fact, that's a pretty common question. But that's mainly because people now realize that our QFTs are only effective theories, valid above a certain length scale. So the question is motivated by pretty much the same sort of reductionist viewpoint that leads people to question how the lawfulness of the Second Law is grounded.