I find it fun to write Cocoa apps too, and I do on occasion for throwaway stuff that only I am going to use. But too many people (including me, at home!) simply don't use Macs. When I have to write a portable app, the choices basically come down to GTK+ (doesn't look native anywhere but GNOME on Linux), Qt (requires C++ plus moc and doesn't always look native either, for example on GNOME), or writing everything from scratch for every platform. While the last choice may be the "right" one from a purist's point of view, the extreme amount of work necessary to make duplicate Windows/Mac/Linux (often plus Android and iOS) versions makes it all but out of reach for anyone but big companies.
