I have no doubt that the delay in announcing has to do with the short term timing of Bruce's cancer. Since the band, naturally, wants to support their album with a tour, they would like to know whether or not they have a singer. If Bruce's prognosis is great, they can plan some dates. If it is good, not good enough to sing but likely to get there, they can come up with a timeframe. If his prognosis is bad, then they can decide what they want to do.
Simply put, they don't want to tour too far after the album, and they don't want to be put in a position to cancel a bunch of dates.
I expect we'll get the full story about this as the tour and album press run develop. It should be the #1 question asked from every interviewer.
There may be another area for this, and I believe they want to announce the album at the same time they announce Bruce's cancer is gone/in remission. Why? Because the global metal community is currently in absolute shock, in absolute awe, waiting for good news. If the album comes out with the good news, or is announced with it, anticipation for it will go through the roof. It could finally be their #1 album in the US.