Earlier quoted context omitted.
You need to consider how the US was originally intended to be organized, which is radically different than what is has become. States were envisioned as practically independent nations which would be tied together by an extremely minimal set of shared rules and obligations. The constitution doesn't really lay out many rules for states at all. What it does lay out is mostly just things preventing states from taking on…
> it makes no sense for the constitution to tell states how they can or cannot vote. This was antithetical to the entire notion of what the United States was supposed to be The United States was originally "supposed" to be made up of a number of slave states where a large chunk of the population wasn't allowed to vote (but their numbers still counted for seat allocation). Everything follows from there - they went to…
So for instance not that long ago in the past we were enforcing eugenics from federal level down. In the famous supreme court case Buck vs Bell, where the US federal government ruled in favor of eugenics, supreme court justice Oliver Wendell Holmes famously said, "We have seen more than once that the public welfare may call upon the best citizens for their lives. It would be strange if it could not call upon those who already sap the strength of the State for these lesser sacrifices, often not felt to be such by those concerned, in order to prevent our being swamped with incompetence. It is better for all the world if, instead of waiting to execute degenerate offspring for crime or to let them starve for their imbecility, society can prevent those who are manifestly unfit from continuing their kind. The principle that sustains compulsory vaccination is broad enough to cover cutting the Fallopian tubes. Three generations of imbeciles are enough.".
Interestingly enough that ruling has never actually been overturned. The reason that eugenics fell out of fashion lay largely with a certain German enthusiast. If not for him it's not unreasonable to imagine the federal level dystopia that we were starting to consider going down. When things are enacted at the state level this gives people the option of getting away from the insanity. When things happen at the top, leaving your country is usually not a practical option.
And so too for the question I was responding to. Should it be legal for foreigners to vote in US elections? Maybe, maybe not. It would probably depend heavily on the state in question. But thanks to our system, that question can no longer be answered by states. In 1996 the federal government made it illegal at the national level.