Apr 16, 2015 - 08:08 AM
Android is open source programming language and that means you can access the android source code. Now having access to Android source code means that you can customize the OS as per your requirements.
It also gives wide API for app developers so that many apps from wide range of tasks can be created.
Both these means mobile/tablet manufacutring companies will have access android for free hence they costs less and thats why it has very wide user base. Also due to this wide user base it is possible to keep Android apps as free and developers can still make money by advertising.