is the poor quality of the senate.
The framers tried their best. But the people voters select for partisan combat in making laws are not the same as the people who you need to perform the non-partisan role at an impeachment trial.
You need a different body to judge a president. An apple is not an orange and an orange is not an apple.
The true uselessness of the system is that Mitch Romney was the first person EVER not to vote with a president of their own party in an impeachment trial.
That is astounding.
The senate's simply not a useful jury.