O motivo pelo qual é obrigatório é que no modo de 64 bits (chamado de modo longo) a maioria das CPUs de 64 bits não oferece suporte ao que o software de virtualização requer para virtualização não assistida.
De wikipedia x86 Virtualization:
To protect the memory of the hypervisor (ring 0) from a guest OS running at ring 1, segmentation must be used. The initial version of x86-64 (AMD64) did not allow for a software-only full virtualization due to the lack of segmentation support in long mode, which made the protection of the hypervisor's memory impossible, in particular, the protection of the trap handler that runs in the guest kernel address space. and Revision D and later 64-bit AMD processors (as a rule of thumb, those manufactured in 90 nm or less) added basic support for segmentation in long mode, making it possible to run 64-bit guests in 64-bit hosts via binary translation. Intel did not add segmentation support to its x86-64 implementation (Intel 64), making 64-bit software-only virtualization impossible on Intel CPUs, but Intel VT-x support makes 64-bit hardware assisted virtualization possible on the Intel platform.