We couldn't prove any form of conspiracy largely due to the fact that the participants didn't think they were doing anything wrong, which is the kind of crazy laws that apply to the powerful that don't apply to the governed (I certainly can't use that kind of logic if the cops show up at my door about something or I get pulled over while driving).

And collusion with a foreign nation doesn't exist as a specific crime, but in a more sane time it would have been easy to see it was what was intended by "high crimes and misdemeanors".