In a Q&A session with My Nokia Blog, Stephen Elop has elaborated on why Nokia's low end range of smartphones, the Asha line (which we recently saw a new phone added to), don't run Windows Phone. Essentially, Elop believes that at such low price points, the quality of a Windows Phone would not be up to scratch, referring to super low-end Android devices that often have horrible performance on cheap internals.
Bringing Lumia smartphones down the price point of the Asha range, or having the Asha devices run Windows Phone, would ruin the integrity of the Lumia and Windows Phone brands. Elop said that Asha smartphones such as the Asha 501 are designed specifically for the needs of low end markets, and the software they run is optimized to deliver a great experience on cheap hardware. Having such cheap devices run Windows Phone poorly is the last thing Nokia wants.
Despite this statement, the gap in price between Asha devices and the lowest-end Lumia device - the Lumia 520 - is quite small. The Asha 501 goes for $99 outright, while the Lumia 520 is available these days for as low as $130 outright. That said, although the price difference is only $30, the Lumia 520 might still be unaffordable for some in developing markets, and thats where the Asha line will be successful.