The SDK is primarily for serious developers interested in developing an app to be available via the iTunes app store as a free or paid app. There is also a developer enterprise program for a company with 500 or more employees that is interested in developing and deploying proprietary in-house applications to authorized users in the company only.
Selling the SDK to Windows users for only $50 when becoming a registered developer costs $99 for the standard program and $299 for the enterprise program just for some that are interested in developing their own apps for their iPhone only, along with support cost for many of the Windows users that certainly would follow just so some Windows users can develop their own app for their iPhone which doesn't gain Apple anything? Along with the resources and cost involved with porting the SDK to Windows in the first place and following support cost, this would be a losing venture for Apple with nothing gained. The more quality apps made available via the app store is a potential gain for Apple - not from the sale of apps which is probably a break even, but from the availability of good apps over time that will likely help to increase iPhone and iPod Touch sales.
Just as the iTunes Store for the sale of music and video is designed and set up to be a break even for Apple with the profit gained from iPod and iPhone sales. Even though Apple develops and has their own OS and software, they are primarily a hardware company first and foremost, so computer sales will continue to be important. And with a Mac including an Intel processor, one can install and run Windows and Windows applications natively on a Mac for the best of both worlds if both are needed.
An Apple computer with an Intel processor is capable of running more applications than a Windows PC natively - all Windows applications and all OS X applications.
I am sure that anyone interested in serious development for the iPhone which the SDK is designed for already had a Mac, or purchasing at least a Mac Mini was not a big problem or a hinderance. And the same for a company with 500 or more employees that wants to make use of the developer enterprise program.
I believe Apple's pride and set ways are the main reasons they are not doing as well as they could be.
Apple is doing plenty well because they are profitable, which is most important. And doubtful that selling the iPhone/iPod Touch SDK for $50 to Windows users interested in developing their own app for their iPhone only would be a profitable venture - not with the upfront cost to port it to Windows and the resulting and ongoing support costs that wouldn't gain Apple anything in the long run, and this definitely would not help sell more Apple computers.