As much as you don’t want to hear it, right now the iPhone and iPhone OS are what’s hot and desirable and easier to design for, thus that’s what people are buying and thus that’s who manufacturers are targeting.
When designing for the iPhone, you know that the user has a fixed screen of a certain dimension and that their hardware will be the same. It’s easier to develop an application when you are guaranteed the specs of the hardware, it’s more likely to run correctly, etc. Since Windows Mobile is an OS and not a hardware/software combo like the iPhone, it is impossible to design for it since you don’t know the screen size of the user’s device, you don’t know their phone specs, etc. There’s no way of knowing if your app will work the same on each device or not.
Beyond that, Apple has created a device where it’s very easy for people to spend a dollar or two on apps. As far as I can tell there is no “App Store” for Windows Mobile, certainly not one that’s as popular or successful as Apple’s. People who use the iPhone tend to be very willing to spend 99 cents on an app, thus developers are attracted by the ability to make money for creating interesting and cool apps, so there are lots of apps for people to pick from so they buy the iPhone/iPod, thus more developers come… it’s a never ending circle.
The same is true for physical gadgets – people who use the iPhone are more likely to spend money on accessories, and it’s easier to design the accessories when there’s only a few models of the phone and iPod and they are very very similar.
Apple is very good at getting people to part with their money. Microsoft is not so.