This is an earlier series by Urasawa, before he had the writing chops with arguably less dynamic art than his later work. So the writing is far more episodic, with little connecting the chapters, or story arcs that last longer than one or two. A publisher can kinda get away with publishing them out of sequence. That being said, it's part of why manga struggled to catch on in the US. Not only was it coming in at the beginning of the worst era of US comics with the market starting to be saturated by Watchmen wannabes and foil variant covers for speculators, it was being chopped up and published piecemeal, with vague notions of making it look more like US comics with monthly floppies of a fixed 22~page length (sometimes necessitating odd chapter cuts).
The choice of material was strange as well, since Urasawa had published both Yawara! and Master Keaton by this point, either of which are far stronger works than Pineapple Army. My guess is the episodic and simplistic action-packed narrative of Pineapple Army was more attractive to US publishers still mostly stuck in the mindset of superhero comics; Yawara especially would have been a hard sell in 1989. Also, at the time, high page count trade paperbacks of comics were still a novelty; even bande dessinees rarely ran longer than 50-100 pages. Manga with its 10-20+ volumes was a pretty big hurdle to publishers.
(Arguably given the fact it was 99% black and white it could have been a good option for pulp sci-fi hawkers like Del Rey or Ace; imagine if they had taken a chance on publishing some of the higher quality contemporary or even older sci-fi works)