People tend to develop biases when they are young and reinforce them when they get older.

If you cut across someone's bias, they tend to see that as your attempt to trick them. And they fall back on a body of knowledge/experience that contradicts your efforts. If you encounter someone without bias, it is easy to seed a bias through presentation of slanted perspectives and cherry-picked information when you are their primary source of truth. But if your audience reflexively distrusts you, it is comparatively difficult to reshape their beliefs. You tend to have more luck playing on their distrust of you.

As an example, if you have someone who is racist and fearful of young, strong, black men, it is relatively difficult to tamp down that anxiety when around a person like this who they find intimidating. It is comically easy for a person that fits the description - but who is otherwise cheerful, passive, and pleasant - to intimidate the bigot with idle threats or make a fool of them by playing against their presumptions.