Volunteer time/interest.. not enough people want to scratch the itch.
The need for multiple binaries - (OS X 10.4/10.5) x (PowerPC/Intel) x (32/64 bit, maybe) - is also a pain. You can cross build, and you can munge multiple builds into a "universal binary", but it's not obvious enough to avoid the same discussions happening on every open source project's mailing list.
Native GTK is not the Apple problem. GTK developers could use Mac application paradigms instead of Windows's and embrace GNUStep/Objective C for GIMP and GNOME instead of implementing their own toolkit. They made the choice of going Windows-like way, not Apple. That's the reason of why "on Windows for years, but still tricky on MacOSX".
Why has it been a dead-simple 5-minute thing to install those apps on Windows for years, but still tricky on MacOSX?
Why did it take, what 7 years, from the launch and immediate celebration of OSX, to get native GTK?
So, without knowing the answer to the above, I guess you could say I'd like it to be more like Windows (ducks).