There are definitely some plusses to developing on iOS. The Interface Builder is very good, and makes it really easy to quickly put together interface ideas, and to switch things around during development. Apple provides a set of good high-level frameworks that make many of the things you want to do a simple matter of a little glue code. And by a little, I mean "ten or twenty lines".
It's also a very restricted, simple platform. The hardware is all standard: you don't have to worry about too many different resolutions, CPU speeds, etc. The software is all standard: you have one (mainstream) language to write in, one API.
Microsoft isn't nearly as restrictive. Just from Microsoft, there are a bunch of different official APIs in different languages with different levels of abstraction. That flexibility can be great because you can choose from support of lots of different OS versions, between native code and managed code, etc. But when it comes to learning how to do something, it's a lot easier for someone to say "Here's the one way to do it" than to sort through decades of history. So I think that there are a bunch of people who find it easier to play around in iOS than on Windows.
Another thing is the iOS SDK is free. MS has released Visual Studio Express for free, but there are limits compared to VS Pro. I'm not familiar enough with VS to know whether the limitations in VSE are a problem or not.
