Well, developers have two good reasons for writing iPhone software:
1. It's their job and they can make money off of it.
2. Whether they make money or not, it's a much more popular platform and their app can be seen by more people.

The annoying thing to me is that there are a ton of awesome libraries for Linux, but the UIs are lacking. I have a strong suspicion that with a separation of UI from development, this would change a lot.