Constitutional Amendments were designed to give each generation the power to decide both the specific content and the purpose of the Constitution; which was not ever intended to be a vehicle by which the long dead dictate the form of government to the living.
Not, of course, that national health insurance costs the original purpose of the Constitution, which is laid out in the preamble, however much one might argue it violates the details of the design.
> Why not allow states to do it for themselves? Broad-brush national solutions do not always work well.
Why not do it nationally? Isolated local solutions do not always work well.
> Labor costs are higher in certain places, conditions vary, etc. Why do it federally? My
National programs can, and do, account for local variations of the type you describe, and state-level programs aren't immune to having to deal with them, anyway.
That's not an argument against the policy, it's an argument that if the policy is desirable it should be pursued by means which include a Constitutional Amendment.