Android phones, apps and tablets

The Symbian platform is scalable, meaning it can run in high end devices like the N95, the N97, the Samsung i8910 and the Sony Ericsson Satio. It can also run in mid tier devices like the Nokia 5800 and 5230. It has a flexibility that none of the others have, mainly because it's been architected from the beginning to be optimised for mobile devices (rather than being a desktop or server OS shrunk to work on a lower power processor). This is why it is the market leader because it is available at a lower price point than any of the others thus opening up new markets for smartphones, i.e. India, China and much of south east Asia.

It's not the market leader because it's been around for far longer, no, that wouldn't be it.

Windows isn't the most efficient or stable OS, nor is it the cheapest.. but it still dominates.