This can be a pretty involved issue. I know some time ago the research showed that iOS owners were spending more money on apps than Android. I suspect there may have been a similar situation with other add-ons, though I don't know. This may also have evened out a lot more, but I haven't seen recent figures.
But let's just go with that for now. It could then mean that even though there are far more Android devices, the greater sales for a mobile keyboard would still be with iOS.
Then there are the issues with the different versions of Android. The latest one has only a small fraction of the total Android products out there.
I have no idea how much those different versions would also require important coding changes, but that is another reason they may have avoided starting with Android.
Since I have never done programming for things like this, I have little knowledge of how hard getting Android working will be after iOS is done. My feeling would be that the basic user interface wouldn't be a big deal (I'm assuming it wouldn't need variations based on different versions of Android). I mean, you put info on the screen, you have sliders, etc - all things I would think would be pretty familiar to any professional programmer who does Android devices.
And then you store the settings and, finally, install them where necessary. Not that I know how to do any of them, but, some things that are somewhat similar in concept I have done decades ago on computers. Maybe this is wildly different in concept.