Why IE can't be replaced by 3rd party browsers just yet

This article on slashdot highlights one of my biggest gripes with the contenders for best Browser. No matter what your non-IE flavour, be it opera, firefox or some homebrew creation you came up with yourself, none of them have the robust page handling that IE has. Sure they have bells and whistles up the wazoo, but what use is Mozilla to an asp.net developer who can't display a page let alone debug it, and not to mention opera's shananigans when you try and do client side error validation. All in all, these browsers are nice in their own way, i've tried most of them and i have to say they have features that you yearn for in IE, but at the end of the day, they aren't a true 100% IE replacement. Perhaps a thought for these developers should be that no matter how many features, plugins and ding-a-lings u cram into ur browser, if it can't display a web page, its useless as a browser.