This comes at about the same time that Apple deprecates Oracle's Java from OS X. Is this a general move against preinstalling third party libraries or unrelated?
Perhaps it signals Apple's belief that both technologies are moribund, if not dying outright.
Perhaps it stems from Apple's claim that Flash is bad for battery life. This can matter even when there aren't obvious Flash widgets on pages you visit. Flash is used for ads and any number of tracking processes.
As much as some will love to pour hate on Apple for this move I think the real failure here belongs to Adobe who has been satisfied to sit on their laurels blaming Apple for victimizing them rather than delivering a compelling Flash experience on mobile devices or OS X (3+ years after the iPhone's release only now is full Flash running on mobile (Android) devices and reports are mixed on the experience).