To be even more specific, it happened because apps dumped DLLs into shared system directories instead of bundling them alongside the app binary. Since on Windows, each app has its own directory, it's easy to bundle in a way that doesn't mess up other apps.
I mean, if all apps avoided writing to the global folders (i.e. reserve it for OS-managed components), there wouldn't be an issue even with the old arrangement.
But not even Microsoft's own products respected that in practice.
