Why HTML5?

To get a level of ubiquity and integration simply not possible with Java/JavaFX, Flash, or Silverlight.
IE’s market share is dropping by one percent per month. (Note: this isn’t so much “why HTML5” as to “why not simply target IE?”). IE’s mindshare is dropping much faster. Microsoft is well aware of this.
Any application targeting 2010 and beyond need to be inclusive of mobile devices.