Like most things in the USA, voting is a local matter, not federal. It would be prohibitive for the post office to manage this across 50 states with differing rules.
States in the US have more atonomy than states in most of the rest of the world. So much so that it often doesn't make sense to compare national US figures to national figures in other nations - education being a good example - https://learn.uvm.edu/blog/blog-education/vermont-eighth-gra...
But the same applies to voting. The federalist system under which the US operates allows communities and localities the flexibility to operate in a manner consistent with local needs and values.
