Probably because it has relatively painless GPU sharing with pass through. As far as I know that sort of feature requires a hypervisor-level VM, which is not something you get with VirtualBox.
Someone correct me if I'm wrong, but I think you can use a KVM or QEMU backend for VirtualBox and that way get GPU pass-through. Probably not out of the box though.
The WSL2 GPU passthrough is more like a virtual GPU than KVM style device passthrough. I believe it's effectively a device specific linux userland driver to device specific windows kernel driver with a linux kernel shim bridging the too. If I recall correctly, the linux userland drivers are actually provided by the windows driver.
