Compatibility. People already have money invested in their products, and while a simpler version of d20 would appeal to some folks, people aren't really going to be wild with the idea of not being able to use their favorite whatever, or having to jump through all sorts of hoops in order to do so. Show what a typical character build would look like using the default d20 rules, and then show what you toss or fold into something else to arrive at the same character using the new system. A couple of different examples like that showing a couple of different types of characters, and I'd be much more likely to get the product.