It is possible that the DNC isn't saying anything on the advice of lawyers, or at the request of the FBI, CIA, NSA, military or other federal agencies, who are undoubtedly investigating the hell out of this. This was a cyber attack by a hostile foreign intelligence agency, not some script kiddies defacing a AOL web page, so there are going to be a lot of people looking into this.
It is also possible that the DNC is still trying to work out what is real and what isn't. A smart way to manipulate this sort of info is to leave 99% alone, and just change a few key things so there is an air of authenticity about it. It would take a little time to properly work out the dif of the two streams.
To be clear: I never said that the email isn't 100% authentic. I am just pointing out that only a truly stupid person wouldn't start asking a lot of questions about this info considering the source.