I’m not sure it’ll make a difference or be satisfactory, but having owned a solo, and a 5900 I think I can explain a “why” - at least for me.
I took my solo and loaded it up with lid and access bag back a while ago - it worked for a 5 day trip but was strappy/heavy for the total cubic inches. I eventually went to a sky 5900 on release and loved it. As others noted, perfect multi day size, and sky mode has always been nice. I used that bag a ton, longest of any pack I’ve owned. But I eventually didn’t love the full beaver tail, center zip back pocket, and ran it 99% of the time in bivy mode with a lid, rarely needed more volume.
I’ve always wanted a “bigger solo” (so have a lot of other people which is why they said they built this bag) and they did exactly that. I think their 5 day archery season, multi day later season description is spot on for the volume. The reduction in straps is noticeable, and simple.
For the record, I’d be cool with a lid attachment capability w/this bag like the solo. But on the flip side, I like a clean easy to get to top this bag offers, and the zipper configuration is my favorite of all their packs. Still plenty of compression, and adding extender straps to attach heads up top is easy.
Not trying to convince anyone with that ramble, but I think it likely explains the “why.” They already have all kinds of different bags covering a lot of size ranges, and it’s cool they finally did the “big solo” which is more of a niche thing, at least to me.