because flash isn't always cross platform. those of us on linux had to wait 2 or more years (I really don't recall) for adobe to roll out a flash 9 from flash 7 for linux. It took another year after that to get it stable.
Also if you are blind how do you view flash websites? it has serious problems with usability for those who are disabled.
it also has usability problems for those who aren't disabled. I don't like flash websites because I can never find my way around. I can't run them through translator's either (an artist I like, who's native language is spanish, website is in flash, I can't read or translate the spanish because of that, and the english version has been coming soon for 2 or more years).