That's the other easy explanation as to "why iOS?"
Note, not picking on you, just Internet in general.
The old saying was "Windows is not done until Lotus won't run", and the DR-DOS case shows it might not have been an exaggeration
Google/Chrome also works hard to support "what's already out there", which is what you mention, but what we're discussing now is building on your supposedly-open-but-really-proprietary platform _against_ other players, whether that's policy or coincidental with constraints.
Microsoft spent effort making sure Windows itself WON'T run on DR DOS before that. They constantly ignored web standards after they won the browser wars (conveniently working well with Microsoft tools that produced non-standard markup, though) ... up until the moment they lost them again, at which point they started to pay attention to standards again.
With their last few releases, Google seems to be adopting this Microsoft style of evil. Some people defend that, and some don't.
Then you ask yourself why the goals were set the way they were. Obvious guess at an answer: because they only want to target "mobile" and Android+iOS cover most "mobile" clients. Had iOS had less market share, I will bet the goal would have been Android-only (modulo advice by lawyers based on antitrust worries in that situation, of course).
No malice anywhere along here, but the end result is not so distinguishable from malice, sadly.