The best apps I've used have implementations for every OS and UI separately. Usually, everyone uses the easier route, but it will only be good enough, not the best. But again, now your app works only on Windows.
I don't even have a Mac yet, so no point in shipping for that if I can't debug it.
If sales are good, I'd be glad to buy a cheap macbook off ebay and port it.
Naturally nowadays this is too much to ask for, so many ship the Chrome Application Platform instead.