That's long standing nationwide practice of never issuing 'receipts' for who you voted for that goes back many, many decades to prevent (or minimize) the ability to sell your vote.
Imagine ACORN or ANTIFA or some other Soros-funded group promising to pay you $5, $20, $100, whatever amount, if you come to them after the election with a receipt proving that you voted for candidate X.
Oh, that makes sense.