For one, neither are available on my phone or tablet of choice today. And due to their proprietary nature, they likely never will be. Apple gave Adobe 3 years to bring Flash to the iOS platform (possibly longer if they were given specs prior to iPhone shipment in mid 2007). Politics aside, locking into vendor proprietary plugins results in the inability to be platform agnostic. The desktop market is pretty stable and not changing much these days, but the mobile computing market is exploding, with tons of OS choices. None of the mobile platforms allow both Silverlight and Flash today, though maybe WP7 will be the first.

On top of that, Flash on the Mac is just a very horrible experience. It's pretty crummy on Windows with Firefox based on what I saw at Mozilla's crash page. In both cases, the proprietary nature of the plugin prevented Apple or Mozilla from being able to fix the problem, and instead they had to work around it by running plugins out of process. And neither company can address any performance issues.

Compare that to the state of HTML and Javascript. Microsoft basically declared themselves the winners in the browser market, and promptly got rid of most of their engineers on the browser team. Web 2.0 came along, requiring better javascript performance. Had it been a proprietary Microsoft tech, things would have stalled out till Microsoft saw it as a priority. Instead, Mozilla, Apple, Google, and others were able to just make their own new browsers with better javascript support, along with building new mobile platforms. And by reintroducing competition, it forced Microsoft to reform the IE team, and finally at version 9, they are catching back up to where they should have been years ago.

Sure, I want to run applications too. I'd also like to avoid giving any one platform support to the extent that it becomes dominant. In the case of Microsoft and Adobe, their definitions of "good enough" don't match mine. And once they hit good enough, they don't bother to keep innovating. Until their track records improve, I refuse to trust them with such vital technologies when the field is still advancing frequently.

[/soapbox]