And i also think its cultural thing though, they most likely have a huge community with people who has a flare for designing things that goes into the Apple-ecosystem, OS X is pretty nice to look at, and therefor people need to up their design principles to make their applications fit the OS, but don't get me wrong, there are a lot of ugly apps fro OS X as well, they just don't get the same promotion as the good looking apps. :D

Apple is a lot about design, also when it comes to development.

Only my personal thoughts and what i believe might be the reason.