Why not using HyperV itself instead of Vbox?
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.
However, when I last spent a couple of sprints attempting to get them working side-by-side, it was a pretty big failure. I couldn't get VirtualBox 6.0 running without falling back to soft virtualization which was painfully slow (booting a Ubuntu box took the better part of an hour).