The problem is that its current operating system requires far too much memory (RAM) to run on older or cheaper smartphones. If the unconfirmed, albeit credible, rumor is true, Android 5.0 (code-named Key-Lime Pie) would require only 512 megabytes of RAM, allowing it to run on smartphones that cost as little as $100.
In trying to expand to emerging markets, Android's OS has the advantage of being able to run on a variety of hardware. In contrast, Apple's single product, the iPhone, may be popular in developed countries but its prohibitive price tag makes it an unlikely choice for mass adoption in the developing world. Part of the reason Nokia still claims sizable market share in India is because it sells phones priced between $50 and $100.
If Android's operating system can be streamlined to run on lower-end smartphones, that would open up a potential market of billions for Google.