I think there are a large number of people that view it as a racist attack on the first black president, with no basis in reality, kept alive for years by DJT himself, and therefore vitally relevant. Especially combined with the reality that DJT was personally sued twice by the Department of Justice for not letting black people rent homes in his buildings, there is a sizable contingent of Americans for whom this matters. You may disagree with the characterization of his motivations, but a lot of folks don't see it that way. If they are right about that, it is one of the more important questions that could be asked (and he still didn't answer it but dodged it twice during the debate).

HRC's emails also aren't as big a deal in isolation but again, like Donald, they tell us something about the very nature of the candidate and provide a map for how they will govern.

Personally I think HRC (emails) and Trump (taxes) are both candidates who couldn't give two shits about transparency to the public (and will govern that way), and that Don is fundamentally a racist (housing discrimination, birther, refusing to swiftly disavow support from David Duke, etc).