It's all about churn. It's new and shiny. Oh you actually depended on those features? You want document format portability? Those questions don't enter the equation. Eventually they dump everything and replace it with something else. The quality varies from useful to bad, but ultimately it doesn't matter to them, it all gets replaced eventually. Because new is better. Stability, backward compatibility, support for open document formats - these things don't sell in Apple's world view.