I'm not sure if you're being facetious or not, but it's because 7 & 8 are much more standards compliant and take way less time to hack on to get working. Plus they have a huge market share.
I think he means why not get rid of IE all together? 7 & 8 are more standards compliant than 6 (which isn't saying much), but they still are lagging behind Firefox, Safari, Chrome, and Opera.
Depends what you mean by 'standards'. 7 & 8 support more of CSS 2.1 than the other browsers.
I believe support for CSS3 is lacking because they didn't want to write to a draft, and then have to support no-longer-standard extensions when the draft changed.
Completely anecdotal, and kind of a throwback to the Netscape days, but in recently redoing our site, the main things that I had to fix to get things working with IE 8 were actually bugs in our HTML. There was only one issue that we had where the fault wasn't ours.
I'm sure that IE8 is still comparatively the bottom of the barrel, but the gap seems to have narrowed pretty significantly. (Incidentally, IE7 still gave us large enough problems that we decided to just set things up so that it degraded reasonably when using IE7.)
