The introduction of the Photoshop SDK was part of the adobe 5.5 announcement which also included the introduction of a subscription based model.
Subscription based is not cheap by any means.
US$95/month x 12 = US$1140 for 1 year of use vs. 1899 purchased. The model could be for some in terms of cash flow.
Although, the month-to-month option can be interesting if you want to try a full version without committing to the full price.
With Adobe now charging you a bit more to upgrade if you skip a version, they’re certainly finding all kinds of ways to squeeze more revenue out of their customers these days.