In the 80s, Microsoft didn’t shy away from developing for CP/M, the Apple II, and the Mac. It didn’t keep its application software exclusive to DOS to give it a competitive advantage. Apple sells microcomputers, therefore we will make and sell software that runs on Apple computers. Hell, when Excel came out, it was a Mac-only product.
I suspect Microsoft is split internally between the traditionalists who want to make Office for iOS (and who may well have already made it on a contingency basis) because that’s where the market is and the Windows 8 people who want to keep Office for themselves.