The fact that there is no impartiality. Pretty much all of the senate democrats, just like all of the house democrats, have already made up their mind to vote for removal.
And conversely?
It's the Dems asking for more testimony. More information from the key players.
It's the repubs rejecting that idea.
Who seems to have made up their minds?
Both sides have. It's why this while thing cannot be trusted. You have a division and you have both sides with their minds made up. As I've said, impartiality is impossible.
The witnesses are irrelevant. If the witnesses provided damning evidence against trump, they'd say "see, we told you, he is guilty". If the witnesses were exculpatory, the dems would say "we don't believe them, and we have all these other people who have said the opposite, so we should disregard their testimony".
This whole thing was a farce from the beginning. From the moment the dems vowed to impeach him before he was elected, though the moment where McConnell and Graham said they would not convict him.