Probably because those 3% are homogeneous, while the 9% decompose into lots and lots of categories, all with specific needs, and all much smaller than 9%.
Thinking about what it takes to make an IE7 user happy is easy; you just install IE7 and bash on your site until it works. Thinking about all those requirements... it's hard to look at that pile of requirements without recoiling in horror at the size of the task... at least, if you want to do anything else on your site, too. Something as simple as a single drop-down menu colored a bit "cleverly" and you've potentially whacked everyone on that list up there except the deaf. Consequently, I disagree entirely with the idea that it's just a matter of using simple HTML or something... it's actually a matter of choosing to give up on everything fancy, and that's a much harder sell.