It's not that Democrats couldn't prove their case, it's that they couldn't even articulate their case. They don't need witnesses, they need a shrink. If you cannot even articulate a crime that you think President Trump committed, no witness can rescue your case for you.
a) Treason
b) Bribery
c) High Crimes and Misdemeanors
They didn't charge him with (a) or (b). They charged him with (c).
The End