How do you say in English " ¿Cual es el motivo por el que no vinieron? " ?

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What's the reason they didn't come?

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Yes, indeed, in one sentence it would be something like the one you made up or either "for what reason didn't they come?" (que yo sepa, el inglés no es mi primer idioma).

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Could it be... "what's the reason why they didn't come" ?

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I will translate the meaning because im not good at espagnol ,so that's mean :why they didn't come?what's the reason?