WhyNoOpenID said: "However, when it comes to making money as a designer, it seems perverse to sacrifice something like the use of Axure for what is, really, a personal preference."
The choice of platform is more than a personal preference. It affects productivity. It relates to existing infrastructure and related costs. And when serving a multiplatform audience, I know from experience that it's better to make a conscious choice to avoid creating a mono-culture in your organization.
I've been using Windows for publishing work since 1993, and I know that I am more productive using Mac-based solutions. I can use Axure RP in a VM running on OS X, but that solution is far from optimal. Using a VM is a good solution for a tool that used occasionally, not one that's used constantly.
As it has been said earlier in this thread many times: The design industry relies heavily on Mac OS, and Axure RP is a unique tool. Putting the product on both Mac and Windows would make Axure a giant in this field.