Guess what? This year one billion people will get a new phone.
How do you test your software on a billion devices and repeat that every year? How can you make sure it works on device A, with firmware B, connecting through carrier C, located in country Z? If you are an old boring proprietary vendor, you can't. It is impossible. You will kill yourself trying to do it. Do not even try. Others did and failed...
The only way to build a mobile platform for the mass market is via a community effort. Open source is the answer for all the companies that died in the last five years, struggling to support more than just 10 devices.
Open Source applies to Mobile better than any other market. The community effect is the "killer factor". It does not only bring quality. It makes a platform real.
Go Mobile Open Source before it is too late :-)