The full report will be out in May. That means it was already compiled and they are waiting for payoffs. No other reason to delay, delay, delay. As with the first report, no new investigation was included since 2017. Anything new can be addressed in a continuing report.
Actually, the reason for delaying the report is so the media doesn't poison the grand jury pools. McCabe almost certainly has a sealed indictment, as do others. When the GJ was done, the report was released. The same will be true of the other reports - indictments first, release later.
While that is a possibility, it is doubtful. There was nothing criminal in the IG Report. It was a 7 month old whitewashing. Lacking Candor is not a crime. The Report avoided all the obvious and provable crimes.