I think it is mostly have to do with company's talent pool and focus.
As far as I can see (I may be wrong, this is my estimate) Google tries hard to devote best talents to 1) ads and Search and it's maintaining 2) Chrome team 3) Google apps and services online and on iPhones.
After these option they try to develop Android as kinda (I don't know what is they correct word, so I am going with kinda) side project because they know the world needs an open platform for competing with apples devices.
The reason because why Android is unacceptable from Google is we all used Google's best products. Chrome and search and we kinda (by default) think about Google as non fallible.
But that's nowhere near the reality, they have limited number of super smart guys, and they try their best to keep core products (search and browser) much better than competitors.
Most of the time when Google does release a new app their iOS apps is better than Android's ones.
We would be okay if Android was from mediocre company. But it is not, it is from Google , which is the best in some areas. But they don't have unlimited talent. They are trying their best to balance.
I am not saying people who worked on Android are not that bright. No I am talking about more broader picture. Of course there some excellent people who work on Android. But I am not talking about 1,2 or three. I am talking about management perspective in broader sense.
This is my personal understanding, I may be completely wrong.