Just because you can type "sudo" on both doesn't make them similar when it comes to making GUI applications. Again, the frameworks are completely different, meaning that a complete or nearly complete rewrite would be necessary.
Again, not more than the one needed for Windows.
Windows has an order of magnitude more users, and probably comparatively even more money than that.
The first step to getting commercially-focused software on your desktop OS of choice is being a desirable market to cater to.
They are using the Cocoa toolkit on OSX - that is a Mac-specific thing and isn't available on Linux.