Why can’t they just pull out the ballots and Show the World??
A prosecutor rarely announces his evidence to the public - I don’t see why everyone is expecting Team Trump to show their cards.
Criminals will often compound their crime in order to avoid conviction. Some examples: witness tampering, bribery, destruction of evidence, false alibi, perjury or any number of other forms of obstructing justice.
This is a lot easier to accomplish if the criminal knows exactly what evidence the prosecutor has.
Also, the corollary is true: if a criminal believes there is no evidence or only weak evidence - he may let down his guard and engage in risky behavior.