If you make a positive assertion (they cheated in the election) then the burden of proof is on YOU to prove that they cheated. If you cannot prove beyond a preponderance of the evidence (in civil court, more likely than not) or beyond a reasonable doubt (in criminal court) then they are deemed innocent until proven guilty.

In most cases, you cannot prove a negative.

Let's use an example that's a bit less nebulous to make the point. If I claim that you stole $100 from me, then the burden is on me to prove that. It's impossible for you to prove absolutely that you didn't steal my $100 without having been under constant supervision. The only way that you can effectively prove your innocence is to provide counters and explanations for my positive assertions to prove that my assertion does not withstand scrutiny. Like if I said that you stole it last Friday, and you could respond that on Friday you were out of town and you have contacts who can confirm that. But without my positive assertions being disproven, you cannot really prove your own innocence. If I just said you took my $100 but gave no further positive argument, you would have to prove that at no point in your entire life could you have stolen my money.

That's why burden of proof works the way that it does. The person who makes the claim has the burden to prove the claim.