So he took to his personal blog to make the case more clearly.
Here are the three big reasons he lists:
HTML5 is already almost universally supported in mobile browsers and Adobe realized that Flash would never get there. “Our goal has always been to obtain the same level of ubiquity for the Flash Player on mobile browsers, but, at the end of the day, it is something that did not, and was not going to happen.”
Apps made browser-based apps less necessary. “Essentially, users’ preferences to consume rich content on mobile devices via applications means that there is not as much need or demand for the Flash Player on mobile devices as there is on the desktop.”
Fragmentation. To make Flash work on mobile platforms, Adobe had to work with multiple hardware makers (Motorola, Samsung), platform companies (Google, RIM), and component manufacturers (like Nvidia). That took too much time. “This is something that we realized is simply not scalable or sustainable.”
He does not mention many of the complaints that led Apple to refuse to support Flash on its mobile devices, like reliability and battery life drain.
He also made the case that Adobe is not killing Flash completely.
Adobe will continue investing in and promoting Flash for desktop browsers, as well as AIR on mobile devices. (AIR lets developers build apps in Flash, HTML, and other technologies, then package those apps to run on multiple platforms, including both mobile and desktop computers.)