In other words, for many important websites, Flash was the JavaScript of today (far reaching, with critical appearances in both the back and front-end).
Business Reasons: Adobe has had a rough decade. The Macromedia acquisition was a red pill (IMHO) for many at the company. The company has been dramatically slow in adapting to a new Web. Several major projects have failed or under-performed. All and all, Adobe has bigger fish to fry than to procure their developer base of a free/low-cost migration solution.
Other Reasons: 1)The nature of proprietary software - a lot has been written about this so feel free to look it up on the Web but basically its hard to just discontinue software without facing several non-trivial legal challenges. 2) The Steve Jobs Effect - or the curse of having a major figure accelerate the demise of a product that should have evolved into an open standard. 3) The Adobe Coldfusion Effect - Macromedia/Adobe has a product called ColdFusion Server that used Flash technology A LOT to differentiate itself, however the transition strategy adopted by Adobe didn't immediately discontinue Flash support for ColdFusion generated reports (for example) and such a direction made developers less likely to invest in a quicker migration to HTML5.
I feel your pain and, at this point, would also like to see Adobe pull the plug once and for all. But we live in a complicated world, for better or worse.