As I understand it, the states that fucked up in the first place don't want to pay for the revote (among other issues). Under law, each state pays for their own primary. Therefore, unless and until FL and MI decide to do that, they can't even get to the point of going to the DNC for permission on how to have the votes count.

But that's just what I seem to recall. I didn't really research the issue so there may be more to it (and, by more, I mean something substantive on the law, not "Hillary cheated" or "Obama is trying to disenfranchise people").

Also, according to at least one news story, contrary to what MEM seems to be saying, the people in Florida aren't that eager to revote anyway:
  • Last week... party officials in Florida had proposed a repeat presidential primary on June 3 that would combine mail-in ballots with in-person voting.

    But [Congresswoman Karen Thurman, chair of the Florida Democratic Party] said the party was swamped with thousands of responses from voters who voiced opposition to a revote, and noted that several counties "do not have the capacity to handle a major election before the June 10th DNC primary deadline."

    "The consensus is clear: Florida doesn't want to vote again. So we won't," she said.