While the backward compatibility argument is compelling, I don't believe that it holds water. There's orders of magnitude less incompatibility between different IE versions than between any given IE and FF or anything else.
IMO, the reason behind IE6 longevity is rather simple - it's the brain-dead "genuine windows advantage" program that refuses installation of IE7 on XP if the latter is deemed pirated. Most of the XP installations in China are pirated, there's very little doubt about that. So there you have it.
Microsoft shot itself in the foot and now they don't know what to do. Perhaps, they should blame pirates again.