Because there is a cap on how much the points can change from year-to-year. Without member approval
DVC could not raise any given day by more than 10%. (At least I think it's 10%--might be 20% but I'm too lazy to go look right now, Hope.)
Some of last year's adjustments were the maximum permitted. You could look at that as an indication that additional adjustments were warranted but not permissible in a single year.
In theory, for every person who loses someone else will win. If they raise December costs and lower May costs, it's simply a matter of some paying more while others pay less. It's really no different than the system we have now where April stays cost more than January. It's all supposed to be based upon sound principles of supply and demand.
As for sales "hurting", I don't see the AP incentive as confirming that. On a 160 point purchase, the 8 AP vouchers are valued at roughly $18-19 per point. And DVC is certainly paying less than that amount to Parks and Resorts for the AP vouchers.
The alternative cash discount is $15 per point. I doubt the AP incentive is costing DVC much more to offer than the $15 cash discount.
On a 100-pt add on the 4 AP vouchers have a value of $15-16 per point. $15 is the cash discount offered so really there's no difference.
It's still a great offer for folks who can use the vouchers. But it's not like DVC just slashed prices by thousands of dollars.