Tip o’ the hat to Gruber for this post:
Imagine if 10% of the apps on iPhone came from Flash. If that was the case, then ensuring Flash didn’t break release to release would be a big deal, much bigger than any other compatibility issues. Since Apple doesn’t have access to Flash CS5’s runtime library code or compiler frontend, they might be put in a position where they would need to coordinate with Adobe to resolve those issues. Shipping a new release where Apple breaks any specific application, even a top seller, is not an issue if the release is compelling, most apps work, and Apple has the option of working with the vendor to help them fix their app. Shipping a release where they break a large percentage of apps is not generally an option. Letting any of these secondary runtimes develop a significant base of applications in the store risks putting Apple in a position where the company that controls that runtime can cause delays in Apple’s release schedule, or worse, demand specific engineering decisions from Apple, under the threat of withholding the information necessary to keep their runtime working.