Here are a few of the reasons:
1. The Web Browser Won
If you are writing a new user-facing application today your default should be to write for the web browser as the platform instead of for a specific operating system. Although there are painful differences between browsers, the web browser on a recent computer is fast enough for most of the applications that people use.
Although desktop applications may be necessary where very high-speed processing is needed (e.g. for an application like Photoshop), most people are doing mundane things with their machines (such as browsing the web, reading email, instant messaging, editing documents) and those things can be achieved in the browser.
2. The Browser Speed Arms Race
And there are performance benchmarks for comparing browsers.
Or is it just a language that's sufficiently different from all the others that it's misunderstood.
And major companies are putting muscle behind these libraries with initiatives like YUI.
Compare that to the offerings in the Flash Exchange which are very commercial and focussed on designers not coders.
5. Adobe Flash is missing on the Android, BlackBerry and iPhone