In his biography, he didn't get along well with Adobe. That's the reason he did it. For all other technologies, Apple creates their own competing product. Why didn't they just do that for Flash (creation side and delivery side)?
The writing was already on the wall for 'applet' based web content I think. It was clear HTML/JS was going to take over on the Web, and Apple were trying to push native apps for as much as possible too.
