So, here are a few reasons:
1. On iOS, if you use the native interface the app would look good. On Android, it will look like crap :-)
2. On iOS you have predefined UI effects and transition, on Android you'd have to do custom
3. iOS is faster, and has a much higher memory limit than Android. Allows apps to use more resources (because it's not multi-tasking)
4. iOS doesn't have a capped app size. Most Android phones have application storage limited to around 10Mb. Android introduced lately support for installing apps on external storage but.. check #5
5. Fragmentation. You currently have to suppor 2 major Android versions (4 minor). Believe me, that really caused me a lot of problems in the past 2 years
6. Screen sizes. On Android, you currently need to support 5 different screen size ratios, as well as over 10 screen sizes. handle that!