Adobe applications are built with a proprietary platform-independent application framework to support both Windows and Macintosh. I've used it in the past to write plugins for various Adobe products and found them very cumbersome to use.
The situation at Macromedia is about the same since they also have to support both platforms. Both companies' product lines are also littered with products acquired from other companies which are at various stages of integration.
With Adobe's buyout of Macromedia, Adobe ends up with quite a pile of mess to cleanup. Now factor in Longhorn and .NET migration ahead and I think you get the picture Adobe's engineering executives are looking at.