Two words: Vendor Lockin
This is not a vague, philosophical reason, as others have noted: being locked into any monolithic platform can result in unilateral decisions by the platform vendor killing your product.
All it takes is an API change, a non-approval, or even a competing offering from the platform vendor to destroy months or years of work, and take your business out.
At Starter, we are passionate about building platforms and tools based upon open source standards, open source, and portable platforms.
IF there is a need to integrate with a proprietary service like Facebook, SalesForce, or iTunes, it is done as a connector/plugin... the core server runs on Linux/Tomcat/Java with various front-end clients written in HTML/JS, WordPress, Swift, Java, C#, or whatever makes sense for a responsive front end experience.
With a portable java/linux based backend, you can move the core app to various Cloud providers such as Azure, AWS, Backspace, AppEngine, and even your own data center -- preferably a hybrid of these with some failover capability.
In this day and age, there is a huge trend towards consolidation -- don't be a victim of the whims of a giant vendor, choose portability and open source and you will win.