In the light of so many vendors coming up with different interfaces of mobile computing, android is bound to lead the market, simply because of its scalability. iOS is popular but not market specific, i.e. iOS phones are not targetted towards the people who can do okay with a mid ranged phone. Also, Windows Phone is wobbly, and too cpmplicated (but probably the best among the three) to use. Windows Phone must generate enough interest in developer society and market, because it probably can change and give android a run for its money.
Other vendors are catching up to the leading companies, and Samsung has ever been losing share value since their design changes, which have stayed the same since 2012. Most people buy samsung because there are more samsung service centres than there are hardware stores, but this ritual is changing because other vendors promise even better things.
I personally like chinese brands and even Motorola, which gives a core android experience.
Comparing market shares by operating system is but one way to think of the smartphone market. Since there are a large number of Android vendors, it's no surprise that Android "wins." But if one, instead, ranks by *vendor*, how do Apple, Microsoft, Samsung, etc., fare?