Well, to be fair, Android is extremely successful in poorer (middle income) countries. For example, only 4% of Brazilians and 10% of Russians use iOS compared to 30% of Americans. iPhones are just plain expensive.
There are many very inexpensive Android smartphones, tablets, and wearables, but no such thing for iOS devices, too.
The U.S. is somewhat of an outlier here, e.g. Norway has signifcantly higher per capita income and 75% Android/ 20% iOS market share.
I guess this has more to do with the way mobile phone contracts work in the U.S. AFAIK in the U.S. the real handset cost is hidden behind monthly rates, so most consumers didn't see they paid $700+ for their iPhone. In many other countries (including Norway) consumers have the choice to select handset and contract separately.
Which is few times more expensive than the phone with contract.
I think the biggest difference is Americans willingness to use credit (debt) compared to other countries.
There are many very inexpensive Android smartphones, tablets, and wearables, but no such thing for iOS devices, too.