I think it's the standard that is set by the original creator of the UI widgets available on the platform, in this case, Apple vs. Google. I'm an iPhone UI designer/developer and you can make a really nice looking app without doing any custom design work just by using the widgets that Apple provides. A good example of an app that was nearly all stock is Tweetie 1 for iPhone and it won an Apple Design Award. Apple has…
My hunch is that we're starting to see a Windows/Mac differentiation here, with Android emerging as the clear winner of mobile and iOS being reserved for the trendy/artistic/quality crowd. That carries over into the apps themselves, with Android apps (like their windows counterparts) being clean and utilitarian, whereas iOS developers are devoting more time to making sure their app looks good.
Edit: The "quality" comment was not intended to suggest that Android lacks it, but rather that those who buy based upon perceived quality would be more likely to be drawn to the iPhone.