Why is android popular?
i. Because android is free platform to develop mobile applications. Android is under the Apache license, which means you can download code and use it for free of cost and make money out of it. No one asks for any royalty fee. Profits margins are very high. That's why many service based companies like Infosys and TCS are also entering into the mobile field.
ii. Android programming is done in java, which is very popular for its simplicity. Any one can learn java very easily and program. Since android is based on java, application development time will be reduced and applications can be released in a very short cycle time.
iii. Since android software is free, the mobile device costs have reduced drastically making smart phones available for even a common man. This is also one more reason to make it the most successful platform for mobile devices.
iv. Android can be used on almost any kind of embedded devices like robots, traffic signals, bmw cars, washing machines, TVs, tablets, notebooks, sony wrist watches, etc.. There is no end for its usability.
v. Google and 83 other companies (OHA) backs android to make it successful by contributing code to android for free of cost.