Hi,
there are basically just two ways IMHO to do multi platform. Use a software framework/platform that supports all target platforms (Windows, Mac and Linux should be in the Mix, too) like Java VM (argh) or Qt (better). Or get two/three teams, one building a software for the Mac, and the other team building the software for Windows, and perhaps a third team building it for Linux. Of course you could set it up to share code using C/C++ which can be imported and used in nearly all programming languages and platforms as "native" code/libraries, but you would have to build the user interface from scratch.
I really love Macs. I switched from Windows in 2004. Before that, I have been doing a lot of Windows programming. I love my Mac because I love Mac OS X. I love the attention to detail. I like that everything looks great (well mostly) and works intuitively. And I love the speed. I love that I can type without software constantly acquiring focus - well Microsoft Software does that on my Mac, too. But you really must not use that software anymore.
For small teams, the only way to do multi platform is using a framework like Qt or Java. But these frameworks typically do not implement all the special features of the OS. They look the same on every OS. But I want a Mac App to look like a Mac App. And it should behave like that. As far as I know Eagle is built with Qt. And, well. Eagle for Mac is not a Mac App. It's a ported, or "shared OS" App.
Another reason: I really know what I am doing when programming for the Mac. I did that for many, many years. I really love Objective-C for it's descriptive nature. And Apple is just doing awesome APIs and stuff for developers. I have been programming my whole life (started on a C64) and I never had so much fun and getting faster results than with Objective-C and Cocoa.
Take the 3D mode in Copper. Copper generates most of the geometry automatically. And it generates textures, bump maps and renders with lights, shadows, depth of blur, etc. This is possible with Windows, too. But you will have to dive into Direct3D, or license a 3D engine, but most of them would be overkill as they are written for games. Mac OS X features "SceneKit". An awesome library that lets you create 3D user interfaces and scenes very fast and easy. You will not have to work on GLSL shaders, Apple did that for me.
Although there is a much wider audience if you support windows, I don't think you get that "sound of voice" to really attract all these people as there is quite a lot EDA software for Windows. But I don't think there is really a good Mac App. And this is what I want Copper to be. THE Mac App for Electronics. And if I succeed with that, the audience is large enough...