Adobe makes money from Flash by selling licenses to the tools. During the long period of platform growth, developer community growth and turnover helped maintain profits through new license sales. Now that the Flash community is shrinking, that revenue is not sufficient to maintain the profits that will motivate Adobe to fund continuing development of the platform. This is a business, after all, not a nonprofit like the Apache Foundation.
Meanwhile, you have companies like Zynga making profits from software built on the Flash platform disproportionate to the number of Flash licenses they purchase. Zynga owes much of its success to the high market penetration of the Flash player, which makes its social games extremely accessible to its non-technical core users.
It's reasonable to place much of the blame for Flash's decline on Adobe for failing to maintain the quality of the Flash Player and for underestimating the shift to mobile/tablets. In light of those failures and what must be rapidly falling Flash tool sales, Adobe has made a decision to change the revenue model for Flash so that they can continue to profit from the platform.