Apparently part of the issue for the original filing to reveal this information was that some copyright holders are getting antsy that as the case drags on, they won't also be able to file civil cases against Megaupload before the three-year statute of limitations expires. However, as Megaupload's lawyers point out, there is no urgency here since the government itself made no move to share this information over the past two years. If it really wanted to share the information it had ample time to make the request and allow Megaupload's lawyers to review and take part in the process, rather than trying to route around them entirely.
I'm guessing the recent successes against IsoHunt and Hotfile may have contributed to the timing as well. The MPAA pretty clearly thinks it can use those two cases to go after Megaupload as well, outside of the criminal case which will continue.