I guess because this desperate racist misogynist lying loony who never has any apparent grasp of what he says and does and no appreciation for consequences is also still in with a decent chance of becoming president of the US?
I don't think being silent about this is responsible. I'm not altogether convinced that being loud about it works either, mind, but in general it's better to have tried to expose the lies, fraud, deception and incompetence, rather than to ignore it. The man is too self-publicising for silence to work.