The main reason is a lack of hardware. Android's success on mobile and embedded comes from the variety and quantity of hardware Android runs on, for this reason there is a large user base to create applications and content for. Android didn't start out with a large ecosystem of applications and content, rather by having so many different devices out there this created a great platform. Android runs on millions of phones, there are Android tablets, Android PMPs, Android Navigation units, Android set top boxes and tv devices, Android game consoles and so forth. Like wise Linux on servers has had success for similar reasons and desktop linux in recent years has had increased success for similar reasons.