The massacre would have never even occurred to the Vegas killer unless he had ready access to some form of extremely lethal technology. 

Those battle rifles were easy to obtain, relatively simple to operate and modify, and were designed specifically for shooting people out to a considerable range. They required no imagination, ingenuity courage or skill. 

In fact, they almost invite this type of mass killing. An airplane or ANFO bomb has none of these characteristics, even though the killer had access to/components for them.