I'd pin the main culprit on ShB releasing and then a few weeks later the three did their conference room huddle on what to do next and Yoshida deciding to can the Garlemald expansion and instead leap straight to the finale. We know how many months in advance it takes for FF14 patches to be planned and developed. Bozja would have been in development before the base ShB expansion even released. They were probably planning on going to Lea Monde. But Yoshida decided to capitalize on FF14's ShB momentum and get to the big ending ASAP rather than delay it 4+ years when the momentum might have dissipated by then. So the third installment got canned and they didn't come up with a different field expedition zone replacement for EW.
I'd say that probably tracks with why Matsuno threw a shitfit and left the project due to 'health issues' (and then had to be coaxed back for the most recent re-release of FFT by basically giving him carte blanche, again, and axing some of the shit that made its way into the War of the Lions version of FFT.)
I think the hope was that Matsuno was going to stay on board and help with the Garlean expansion that never happened by planting the seeds and handling that, while Yoshida and other members of his team handled the Cosmic Twitter Bird and crap with the Ancients.
It explains the lack of a field ops zone; it likely would've continued with the Bozja/Empire storyline, likely in/near the Garlean Empire or tying up loose ends since a) they don't like tying 'optional' content with main game content (something they seemed to have backed off of) and b) the geographic realities of where the Bozja main players ended up.
As a sidenote, the segment of the EW storyline where you're basically part of a massive military op always felt odd for me given the buildup and then relative let down where you're basically just doing mop up duty, which was necessitated due to the shit Zenos was pulling. It really felt like it was going to be a prelude to something else.
You can see why there was a narrative vaccuum leading up to DT, too. A lot of the seeds that would've hopefully been planted by Matsuno or had him overseeing things were instead thrown helter skelter and, again, we had inexperienced folks taking the reins and the senior devs being split.
I really don't understand why they continually do this, especially since XIV is the 'cash cow' for them right now. There's at least 2 expansions where they crammed
way too much shit in and/or scrapped things because it was exceeding the scope of their project (Stormblood and Endwalker.)
You can't say that they
really couldn't have milked more Emet-Selch (let's be honest, this is the fucking reason why things got flipped hardcore with EW: we needed more of the breakout character audiences love!) and given themselves room from the Empire story by stretching out the Ancients storyline while continuing it? That approach, using a completely unrelated world that was fully fleshed out, was only the creative highpoint (Shadowbringers) so I guess I understand why they'd shy away from trying that again.