There are a few layers of problems with supporting iMac:
1) Apple's "hostile" EFI implementation. I say hostile because Apple officially does not support any form of Linux on Mac, including Android. Their firmware can cause problems, see the many support pages on Ubuntu and Fedora on Mac, and those have thousands of users and developers collaborating.
2) Apple routinely changes hardware in the iMac, there are several generations, and supporting each model with Android would be very difficult right now.
3) Apple uses custom firmware in things like the GPU, that would mean even if you get everything right using the same components one-by-one to test on a PC, it still may crash/screwup for various reasons.
You're talking thousands of dollars worth of dev time to get a finely-tuned version of Android, and even then, it would probably still be buggy. A Mac is the worst device to try and get Android to work with, and that is in part on purpose by Apple.