Because:
> you can use the editor of your choice and the terminal instead of Xcode.
This is possible with Obj-C, but using Xcode is a far superior experience to RubyMotion and any editor.
> you can use the interactive console to debug your app or to live test new code
Ditto for lldb in Xcode
> you get all the great wrapper gems like BubbleWrap which are heavily influenced by the "rails way" of doing things
There are plenty of powerful 3rd party Obj-C abstractions too.