Android started succeeding before it had an ecosystem. Establishing a viable app ecosystem actually took a long time.
The problem for RIM, Nokia, Microsoft and others is simple really: they fail to align their business interests with all the other stakeholders in the market. Explosive growth like Android's comes when you get a sudden synergy of different stakeholders whose interests all align. It's not one thing.
* Customers want more choice in phone
(bigger screens, different styles, lower cost, etc.)
* Carriers need an answer to the iPhone. Something where they retain
some tiny piece of control
* OEMs desperately need an answer to the iPhone
* Developers want something more open, with more control and less barriers
* Media distributors want some other channel than Apple / iTunes
Basically, Android was a win-win-win-win for Google, carriers, developers, OEMs and customers, and it came at just the right time when all these needs were
urgent. The common theme you see in all these other players is that they are not open, offer less choice, and they are trying to force others to cede control (license our software, no you can't have the source, no you can't customize it, etc).