Because of a lot of existing internal tools that directly call and manage VBox to set up local testing environments and stacks.

It's a planned initiative for the future, but it means a fundamental change in a number of tools and would require a rather large set of rewrites.

On top of that, out of the large pool of users, only a small handful would actually need to run Docker side-by-side with these tools, meaning that it's a huge rewrite to allow a small number of people to use Docker on their desktops. That ultimately means it's getting very little traction and keeps getting pushed down the backlog.