Trump can't order Postal Service to put limits on mail ballot delivery, judge rules
(www.scrippsnews.com)
When the US Constitution was written, it was replacing the Articles of Confederation. The Articles provided for a very weak federal government, leaving most of the power with the States. Those States considered themselves very independent from one another, essentially their own countries.
In order to get the Constitution ratified, with a stronger federal government, many powers still needed to be reserved to the States. Elections was a big one of those things, and so it is clearly defined in Article 1, Section 4, Clause 1 of the Constitution.