Obviously because Apple is rich as hell and can pay them off to release it on iOS. With all that money they can pretty much command anything of a lot of companies...
a) Samsung, which commands an impressive majority in the Android market, isn't a larger company (which it is)
b) Apple would bother "paying off" Ubisoft for the exclusive rights
c) Samsung doesn't have a near monopoly in South Korea (where it's based), the country in which eSports and all-in-all non-shooter competitive gameplay thrives the most (which it most certainly does)
In short, there's no good reason for the game not to be on the Android. Perhaps it's just makes sense to port it for iOS and then to Mac (which is a natural step) before porting it to Android. Either way, I definitely support an Android version.
It is in no way a natural step to port anything from iOS to Mac OS X, they are completely different operating systems, Mac OS X is still based off BSD which is Unix based and shares the majority of it's coding with Linux on which Android is based, Android and Mac OS X are the closest related of any operating system in 'major' usage (Linux and BSD themselves are not considered majorly used).
I'll add that an easy thing to do would be to fix the touchscreen support on Windows... there are an increasing number of windows tablets that can run the game just fine, but the way the client hooks the mouse doesn't work well with touchscreens. This would be relatively easy to fix and then at least you have support for all of the "full" windows tablets.