Why is Hardware Assisted Virtualization disabled by default?
I believe it is for security reasons. A rogue hypervisor can install itself and then run the main OS, the main OS can't tell that it's running under a hypervisor (sometimes considered ring -1). It could potentially be the ultimate virus. So you have to enable explicitly if you know you want to run a hypervisor.
I believe another reason is power-efficiency, as shutting down any parts of the processor that do not need to be used will use less power, which is especially desirable on an laptop.