Why offer an extension now and not around 2042 and why not offer extensions for the others?
Rumor is that
DVC was having to exercise ROFR more and more on OKW contracts, and that there wasn't much of a market to re-sell the points. Now they can sell the points as full 50-year contracts.
Having been around these boards a long time it's fair to say that some owners have expressed an interest in extending. Of course it's hard telling how the numbers were impacted by Disney's price point. As others alluded to, I think most people anticipated paying less for something they can't use for another 34-35 years.
As for the other resorts, I can think of at least two reasons why they have not been similarly extended:
1. BCV, BWV and VWL are all combined DVC and cash resorts. Extending those contracts beyond 2042 would require the cooperation of the Parks and Resorts division. And right now they may be unwilling to commit to keeping the resorts in place beyond 2042.
As much as some of us may consider those locations to be iconic Disney properties, they aren't going to stand forever. Whether it's 40 years from now or 140 years, Disney will eventually demolish and re-build most of their resorts so that they can continue giving guests the sort of fresh, modern accommodations they expect given the premium prices charged.
2. If the primary motivation for the extension was to encourage add-on and new sales at OKW, the other resorts just don't need that boost. DVC typically has to maintain a waiting list for add-on points at BCV, BWV and VWL. If Disney simply lets the contracts expire on 2042 and intends to keep the resorts open as-is for another 50 years, they will have little trouble reselling the points at market prices.
That's not to say that we won't see extensions at the others. It certainly could still happen. But given their popularity, I'm guessing the price point would be even higher than OKW to extend.