Hello, Abo—Whether you choose "don't" or "doesn't," the sentence will be ungrammatical. "Her" is the pronominal subject of the question. As subject, that pronoun must be in subjective (nominative) case: "she." The correct question is "Why doesn't she?" You can say, "Shedoesn't because . . .,"not "She don't because . . . ," "Her doesn't because . . . ," "Her don't because . . ."
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