I draw two possible conclusions as to why the DNC would refuse access to the intelligence agencies, and unfortunately both are plausible:
1) It wasn't the Russians but really Seth Richards (I think thats the name) or another internal person who performed this. Obviously Wikileaks has promoted this narrative, and the untimely death of Seth Richards will likely leave this as forever unconfirmed.
2) They know the intelligence agencies are just as likely to leak anything nefarious as anyone else is, and thus they don't want to risk that exposure. Unfortunately, this can't be downplayed. As has been shown, even the head of the FBI wasn't against leaking investigation details to the press. If the FBI head will do it (without consequence I might add) then why wouldn't any other member of the agency.
I'm open to other possible conclusions as to why the DNC would explicitly refuse FBI access to a server that was supposedly hacked by a foreign government. Sys, why do you think the DNC didn't turn over the server?