I don't agree with their approach, but right now it's kind of the only deduction to make. I agree with you that there's absolutely no problem with them being honest, just coming out and saying it that "hey, we have a new album, and we'll release it at a date to be determined sometime after Legacy III." Some people would be grumpy, sure, but I think we all realize Maiden is a business and dumping an album to the public you can't monetize with a tour is something they're not going to do. On the other hand, they could be keeping it a secret just to keep the speculation going, keep people talking about the band.
Yes, the point of lifting the secrecy would make a lot of fans happy. But the way Maiden as a band and individuals are so detached from social media, the fact they've got nothing left to prove and no need to go the extra mile to please at this stage...I don't think they really care, to be honest. Like my first statement, this is just speculation based on my observations of the band. Maiden haven't been a "fans band" for a long time, maybe ever. They don't try to have a relationship with their fans, and at their age now, I TOTALLY GET IT. Nothing wrong with that. I think keeping on recording and touring is enough for them. It might be frustrating, but there isn't really anything wrong with that. Just my take on things following them fairly closely for 30 years.