VirtualBox and VMware Workstation (and VMware Player) are "level 2 hypervisors." Hyper-V and VMware ESXi are "level 1 hypervisors."
The main difference is a level 2 hypervisor is an application running inside an existing OS, while and level 1 hypervisor is the OS itself.
This means that when you enable Hyper-V, your Windows 10 "host" became a virtual machine. A special one, but nonetheless a virtual machine.
So your question would more aptly be: "Why don't VirtualBox and VMware Workstation work inside a Hyper-V virtual machine?" One can answer because as a VM, the Intel VT-X instruction is no longer accessible from your virtual machine, only the host has access to it.
QEMU works because it does not do virtualization but emulation, which is completely different and explain why QEMU is painfully slow. Virtualization is the process to run a complete isolated machine inside another, but with the help of the processor. This requires the virtual machine and the host be instruction compatible.
Emulation is the process to run any machine inside a running OS, there is no platform restriction, and is why QEMU can run an ARM machine on an amd64 platform.
Note: QEMU has 2 operating modes:
- it can work as an emulator, this is this mode explained above
- it can work as virtualization software with the help of KVM if the guest architecture is compatible with the host's and if the VT instruction is present of course.