But on Vulcan, handguns were enough to destroy Nero's drill. Surely not every handgun on Earth (or even every armed shuttlecraft) is tied to a network that can be disabled by knowing the defense codes?
We might argue that Nero first disabled Earth's main defenses against starships (orbital fortresses, surface cannon) with information warfare so that he could enter orbit, and then tried to be as quick about it as he possibly could before his drill was destroyed by lesser means. Drilling through some fault line next to San Francisco might be a good idea in that respect. Plus it would make Earthlings hesitate with lobbing nukes against the drill, because those would then kill San Francisco as well.
It would still be a somewhat desperate attempt. Nero wanted to avenge himself upon the entire Federation, but his tactics only barely got him through the destruction of Vulcan and failed in the destruction of Earth - not because of the actions of our heroes, but because of actions any random goon could have taken against him. Why didn't Nero come up with something more workable? He had a quarter of a century to think about it, after all.
We're thus left to think he wasn't particularly resourceful. Which might not be a big surprise, considering he was stranded away from his home and culture, which furthermore had ceased to exist at all...
Or then we have to think Nero did have the upper hand (perhaps because red matter gave him unlimited power against all attacks) but something strange happened twice to his defenses, causing his drill to become vulnerable. But that doesn't appear likely from what we see.
Timo Saloniemi