Because too many people treat politics like a sporting event. You root for your team no matter what, and against the other team. You have to do it this way, because if the other side wins that means your side loses.
So there are too many people who view Trump as “Their Guy”, and are “rooting” for him. Anything they hear that might portray Trump in a negative light (like a criminal trial, for instance) must be the Other Side trying to cheat to win unfairly.
I remind people that Roger Ailes was Nixon’s media consultant, and the lesson he learned from Watergate was that Nixon could have gotten away with it if the media was more sympathetic. He then went on to be the CEO of Fox News. That’s no accident. There is a direct line from Nixon to Trump, and Roger Ailes drew it.