For a start, the list of operating systems Hyper-V supports is an order of magnitude less than what VirtualBox supports. Likewise for emulated hardware, like 3D as mentioned a number of times here. The GUI is also much better on VirtualBox.
And Windows many times forces HyperV onto you, taking exclusive control of the CPU's virtualization features, thereby forcing VirtualBox to either use Hyper-V as a (terrible) backend .... or not run at all.
And Windows many times forces HyperV onto you, taking exclusive control of the CPU's virtualization features, thereby forcing VirtualBox to either use Hyper-V as a (terrible) backend .... or not run at all.