Deshalb bist du nicht hier.

how would one say “that’s why you’re not here”

Das ist, warum du nicht hier bist.

I am not a native German speaker; however, I am quite confident that “Deshalb bist du nicht hier” is better translated as “That’s why you’re not here.” than as “That’s not why you’re here.” The placement of “nicht” preceding “hier” implies that the person is not present. The alternative provided by dlobok, “Das ist, warum du nicht hier bist” implies the same thing, although it is less ambiguous. The given translation “That’s not why you’re here” indicates that you are here but for a different reason and therefore is an incorrect translation of either of those German sentences. Suppose I am in a meeting and I offer to fetch coffee for the others in the room. Someone might say, “no, that’s not why you are here.” I am not sure how to say that in German, but both of the two German sentences in question clearly imply the person is NOT here. Possibly something like “Es ist nicht, warum du hier bist” could mean “That is not why you are here.”