Having discussed this issue with friends at Microsoft, they apparently spend astronomical amounts of time, money, and effort to maintain backwards compatibility while still adding new capabilities, functionality, and toolkits. They know that this is a key reason many organizations rely on Windows.

I have no specific insight into decision-making in open source organizations, but given that the resources they're working with are several orders of magnitude less it's not surprising to me that they sometimes end up having to choose between stability and new functionality. (While I personally prefer projects towards one end of that spectrum, I'm happy that different projects have different values for other people.)