I suspect that it's part Ellis wanting to come up with a big woo-woo ending for it, given that it's one of the series that made his rep (Transmetropolitan also had some delays in its final issues, although of course not nearly as long), and part resentment at people perenially asking him when the series would finish. Funny thing is, when #26 came out, I hadn't seen a new Planetary issue for a while; I read it, thought "Well, that's sort of a blah ending for the series, but whatever", and stuck it on the pile.