The most obvious ones involve sticking with the 2066 end date. All new associations dating back to SSR have been 50 year contracts, so there's every reason to believe a separate Poly association would run through 2074. Rolling them into the current association allows DVC to re-sell 8 years earlier.
Separating the associations effectively means they're separate forever. Come 2066, they're back to trying to sell points for Studios + bungalows only, with a superior facility off in the distance. (Admittedly DVC has 4 decades to figure this out, at which time the longhouses would be 90 years old. But the presence of two separate associations places some limits on how and when they can make massive changes to the resort.)
One association gives them wiggle room to fix the bungalow point fubar. At a minimum, it gives a lot more owners 11-month access to the bungalows. Even better, develop a plan to reallocate so the bungalows are more reasonably priced without the Studios taking the entire hit.
Separate associations sets PVB1 up as a second-class citizen value-wise. DVC still has to deal with 40 years of resale price pressure, ROFR and foreclosures. My sense is that a lot of people who own Poly aren't 100% thrilled that it's studio-only, but accepted that limitation because there was no other alternative. PVB2 would be the alternative with more room sizes and views. BRV and CCV differ in that both at least have the traditional options of studio, one and two bedroom. Both have appeal depending on personal preference for location, decor, room size (BRV larger villas), end date, etc. It sure seems like PVB1 is bound to suffer in the shadow of a PVB2.
Aside from the ending date, that's a lot of little things which are easy to dismiss as "eh, DVC doesn't care about that." And that may be true. The choice of wording in the press release didn't do anything to clarify.
But circle back to the question of why. If DVC has an opportunity to sell this new tower for only 42 years, merge all of the Poly points and units into a single association, raise occupancy of the bungalows and market new buyers on the combination of new tower + bungalows + "largest Deluxe Studios" in DVC, why not go that route?