The web is not built for IE6, only those sites and companies that do not invest enough time and/or money in the web side of development are made for IE6. And generally speaking, that is rapidly changing.
Most people are only going to want one browser.
With a few of the major banks insisting on IE, either deliberately, or by poor design, that makes up most people's mind, so IE wins. Obviously, many other sites are IE only as well, again probably due to poor design.
IE patching comes under the standard (and very good) Windows Update process, so tends to stay fully patched (except those scumbags too tight to pay for XP (which comes preinstalled on most machines anyway)). So for majority, IE wins again.
IE7, comes out with decent security, which is a good thing. Annoying, but a good idea. I don't like it (mainly due to rendering problems), due to sticking with it for over a year on Vista. But soon, I won't have a choice - only options will be IE7 or FF2, so again IE will win (slight lie, usually have both installed, but tend to only use one, except for testing)