Android is mostly similar to Symbian in terms of it's offering of features which iOS and Windows Phone lacks. If you look at the newest version of Symbian, it looks almost identical to Android (look up Symbian Belle for reference). Both iOS and Windows Phone are very easy to use and are very fluid. Windows Phone is quite enjoyable, but for app lovers, you might want to avoid it.
The reason why Android is dominant is mainly to do with how many devices run on it. Yes, Froyo and below aren't the most appealing, but Gingerbread, Honeycomb and Ice Cream Sandwich is a large improvement. At the moment, Android is regarded as, "the modern version of Symbian". It offers what Symbian does but with more flexibility and more apps for the users, developers and manufacturers. Plus, Android devices are available on super powerful hardware, if ICS is delivered onto those powerful devices, they could be the fastest phones out there (if done right).
Why Symbian fell so much was mainly to do with how Nokia treated Symbian. While Android and iOS were innovating and refining to provide better experience. Symbian almost stayed the same with little difference over the recent years which lead to it's downfall. People were getting annoyed because Symbian felt old, ugly and it just didn't feel as responsive as the other platforms did. Only now, Nokia is trying to refine Symbian (hence the Belle update) but even then, it doesn't offer anything a lot better than what Android has to offer. Plus, most manufacturers have dropped support for Symbian too.
As Nokia has said, well, Elop did, Symbian (as well as Meego) is a "burning" OS. In other words, it's becoming a dead product. Elop has announced that Nokia is slowly moving away from Symbian and switching to Windows Phone as he sees it as the future of smartphones. If you look at the newest Nokia phones announced, Symbian is mostly just low end and mid range phones. Nokia is using Windows Phone for their main high end flagship devices (look for Nokia Lumia for reference).