I think the reason most apps find the iPhone first is because:

a) Most app developers today are coming from the Silicon Valley/US trained education system. In the US, Apple had unprecedented marketing freedoms and strong exposure. Its userbase is usually more affluent, less tech savvy (in comparison to their EurAsian counterparts), and more likely to have disposal income to spend on appsand content, and will sacrifice the freedoms for ease of use.

b) Since the app craze is kind of new, those developers choose something local to their market with a low barrier of entry. This allows them to save on localization, testing, and marketing. Most development houses try to target their local market's dominant OS first, and the large iPhone buzz has spread to other locales, but the economy of scale and audience reach will be more important as smartphone use in the US becomes ore ubiquitous across a wider dynamic of the demographic. We are only just seeing other markets and lower income consumers demand popular apps and services outside the US. This is right on time with Nokia's cross platform strategy.

The real question is why is the ease of development more important than the addressable audience of targetable devices? Why consider an iPhone or iPad app, which will likely share some code, but is trending toward custom UIs for each device and will require at least some extra dev time for the larger screen, when you could use Qt and address Mac OSX, Win7, Linux, Unix, WinCE, WinMo, Symbian, Maemo/MeeGo, or even Silverlight for Symbian, Win7, and Win?? Is there really any benefit to focusing on the iPhone period while ignoring other larger OSes?! Unless you have an app localized to the US, I see little in that regard.