It’s not just software
Software is notoriously behind schedule, but my experience with book publishers tells me that it's not just software that has this problem. Starting in 1999, for example, F. Paul Wilson wrote a series of five novellas about a future in which almost-human genetically-engineered beings called "Sims" exist and cause the usual moral complications and intrigue that you'd expect in a work of science-fiction. He eventually consolided these novellas into a single novel which came out in 2003. The fifth of these novellas ended up being delayed for some reason and was actually just published this year, at least seven years after it was completed.
Just like there's usually a good reason why software is delayed, I'm sure that there's a prefectly good reason why the fifth Sims novella was delayed that long. I can't think of what it could possibly be, but it's sort of reassuring to see that big, unexpected delays aren't just something that you see with software.