Because if they actually release it they can't either...
(a) fearmonger about a nothingburger in order to undermine confidence in the election, or
(b) wait to spring an actual vulnerability on the public until it's too late to do anything about it in order to undermine confidence in the election (or indeed, to exploit it to undermine the election itself),
...depending on what the contents of the report actually are.
In every case, spreading FUD about a mysterious secret problem serves the ratfuckers' interests more than being forthright does.