As humans, it's easier to reject or ignore evidence that challenges our established position or opinion.
Cognitive dissonance describes how it takes a lot more mental energy to hold two conflicting ideas in one’s mind simultaneously. For example, if we believe that exercise or diet are important for our health, but want to skip a workout to watch an episode of our favorite TV show or eat an extra cookie, we feel guilty and conflicted. Being in this state is uncomfortable and something humans avoid at all costs – we don’t like the discomfort!
And the theory of motivated reasoning explains how easy it is for us to default to the information that supports our preconceived beliefs.
For example, it's easier for ardent Trump supporters to downplay his inappropriate behavior towards women than it is to accept this information and adjust their opinion of him accordingly. Having already committed support publicly, their reputation is at stake and the need to construct a motivated reasoning is high.