We originally built the software for these systems in a cross-platform manner, targeting Mac, Windows, and Linux using a C++ codebase. About eight years ago, we realized that as a small team we needed to focus on one platform in order to accelerate development and take on our much larger competitors.
After a lengthy evaluation of the Windows, Mac, and Linux development environments, we chose the Mac and Cocoa (despite none of us having much experience with the Mac before that). We rewrote our entire control software in Cocoa seven years ago, and looking back I feel that it was one of the best decisions our company has made.
That segmentation is very similar to hobbyist 3d printers.
Looking at the solutions they sell my guess is that the system is expensive enough that adding an iMac rather than some other consumer PC doesn't change cost too much.
At that point they probably picked the platform they were most comfortable developing on. Going back to the first blog post shows that the author prefers Cocoa for developing GUI's and the bio states that he is CTO of the company.
Again, all speculation, but thats how I'll rationalize it until he tells us different.
Don't know about robotics, but OS X is architected to be able to handled real-time audio. So, I can certainly imagine (but don't explicitly know) that it has better responsiveness guarantees.