This guy says that Flash performs consistently across platforms, when it doesn't. That's the main reason for the whole argument, Adobe neglected Macs for years. They did not improve things that could have been improved without Apple's help. Like http://blog.kaourantin.net/?p=82
And why should Apple devote resources to Flash when they're working on a technology that does the exact same thing in WebKit, canvas, etc..? Flash is not a standard, no one agreed to improve it or support it. They only agreed to support HTML/JS/CSS web standards. No one owes Adobe their support.
Microsoft could ignore Flash and push their own Silverlight instead. The lack of cooperation might make Flash inferior over time, but that's the risk Adobe took by not agreeing to become a standard when offered the chance.