The business logic for your app should be written in a platform agnostic way, and will be trivial to port.
...different platforms have different optimal workflows, and capabilities. This frequently drives changes into what you would think of as platform agnostic code. This is especially true of games but is true of most software. The effects of this can vary from just having a bad port (maybe a non-natiave feel, or just plain a kooky UI), to needing to re-write large parts of the "agnostic" code. This can be costly, and time consuming. Also if you have future versions of the products you need to decide if you want to port these changes back to the orignal platform (or platforms), or hold them apart. Both have their own sets of issues.
...even code that can be made platform agnostic isn't always as simple to write or as fast in platform agnostic form. For example use of CoreData on OSX/iOS is very platform specific, but it is tied to how your objects persist across executions, and even how you represent the objects. It can save an enormous amount of effort (save/load is trivial, undo/redo can be close to trivial, and so on). When it is the perfect fit as much as a third of the code you would normally need to write goes away.
Or if you look at Android, writing the "platform agnostic" part in Java gives you garbage collection so you spend very very little time hunting down memory leaks (you might end up with a few places that forget to nil out a pointer and end up pinning down extra memory for too long, but this isn't as common or painful as memory leaks in C/C++...). No debugging pointers that now dangle into the wring types of objects or to system heap structures. That can safe a whole lot of time.
However a platform agnostic core (business logic, or game play engine, or whatever) won't be able to use any of that. You have to restrict yourself to the intersection of what every platform you want to port to will have. I would be surprised if it cost you as much as having to write it twice, but not if it cost you a good 33% more then writing it platform specific.
Then you have the platform specific (UI?) part of your application. Could be pretty small for something like bug tracker, could be very large for a game or maybe a bike ride activity tracker. If making the core agnostic costs you 33% more, and then doing the platform specific part is significant the new platform has to be a very large percentage of the original platform's revenue before it is worth doing vs. making the faster, cheaper, but less flexible core logic and then moving on to a new project (or the next version of the current project).
I know this is sad when the platform you love is the underdog, but economics isn't called the dismal science for nothing.