Krita in fact already builds, using macports, but it doesn't run well at all. There is a lot of work to be done before we feel we can say "Krita works on OSX, here's the download!"
In particular, there are these issues:
* opengl drivers. Krita uses opengl to optimize the display. Every combination of os/gpu/driver gives different problems.
* tablet support. Qt's tablet support is not very good, so for 2.8 we implemented it ourselves. Twice: once for Linux, once for Windows. It would be surprising if we didn't get the same issues on OSX, so that would make a third implementation...
* dependencies. Krita uses quite a few libraries that are not by default present on the system, and we need to make sure all of them build, work and then are packaged with Krita.
* finetuning. OSX users, rightly, are a picky lot! Just having a gui that is pretty much the same as on Linux doesn't cut it: we need to spend time figuring out the differences, platform conventions and then special-casing for OSX
* performance. Every os has its own bottlenecks -- Windows has a very slow file system, for instance. Optimizing will need time.
* packaging. I've done this before, and packaging for OSX is a whole different game than on Windows or Linux. We'll have to start from scratch if we want to produce a nice dmg.
All in all, given that we've got experience with porting to Windows, I am estimating that doing a good OSX port won't take quite so much time and effort... But we easily spent over a year trying to get a Windows version of Krita that people could trust and would like!