First of all, Android is much harder to develop for than the iPhone. This is because the Android ecosystem is so dispersed. There are many more phones, many more os versions, and many more carriers to support. For a resource-strapped application team, iOS is a simpler choice and a quicker win.
Also, there is, what I would say, less of an app culture amongst Android users than iPhone users. I think this because I develop and work on a team that develops native apps for both platforms as well as a cross platform mobile web app. Our iPhone app does about 50 times better (in each category -- downloads, usage, and revenue) than our Android app without exaggeration. Part of this, to be honest, is our iPhone app is better, and that is partly because of my first point. Even in our mobile web app, though, we see about twice as many iPhone users.
Now, I'm glossing over a whole slew of details here, but the net takeaway for me is, if I were a start-up looking to make a dent in the smartphone market, I'd start with iPhone.