Because this is accessible to anyone with a current-generation browser – including those on phones and tablets that either don’t have Flash, or can’t play Flash apps/games at an acceptable performance level. HTML5 opens up the possible market in a big way.

You say that, You are probably true about current state of HTML 5. But it is an attempt, to give browsers power, to display rich content and animation without any third party addons like flash. So despite the young age of HTML 5, the examples are astounishing.

If you have doubts for HTML5, then google for webgl, and check it out some of examples, and see that HTML 5 & webgl supported Graphics Card accellerated 3D&2D graphics long before adobe tried to support withflash player version 11.

Additionally, using de facto IE domination in browser market, is not helping your argument, since IE is the richest source of problems in the cross browser compatiblity area since all the time. IE itself a problem to todays internet world. Nowadays many and many people quit using IE and switch to a more agile and standarts compilant browsers like firefox or chrome. So using a problematic argument to support an other problamatic argument is not helping at all. 

Sooner or later, HTML5 will be the standart in all browsers and performance issues will be overcame and tools to create rich content will be much more. Keeping other aspects of HTML5 such as client storage, easy embedding of audio and video, the new internet era will consist of HTML5 powered products.