I've read a couple liberal-slanted books on weapons laws, which of course makes me an expert
The general rationale given for weapon bans is that if a killing isn't premeditated, i.e. the attacker is just mad at the victim and lacks self-control and is using any weapon available with no real prior training, a gun will be more lethal than a knife, which will be more lethal than bottle-as-club or bare hands. Statistically.
They neatly ignore all sorts of things, like why the aggressor was upset in the first place (it probably wasn't your mere existence), or why you didn't leave when it was obvious you were about to get into a fight (pride).
The bottom line is that a jerk with a knife is more dangerous than an untrained jerk without a knife, and that's good enough reason for most politicians to try to ban them. Which is ironic, because those politicians also typically support learning martial arts that can seriously disable or even kill an attacker.
Another possible reason for this stuff is that the media is so saturated with armed violence that for anyone who doesn't stop to think about these things, guns and knives are, at a young age, associated with unlawful violence. I think that's the best explanation.
Once that initial association is made, unless it's challenged immediately, it grows legs. Other supporting justifications are made: "nobody _needs_ a 4" knife", etc. By the time politicians get elected for the first time, they've built up an entire list of BS reasons that aren't any good, but it's nevertheless extremely difficult to disabuse them of those reasons.
The above statistical-danger-of-weapon-x is one of those supporting justifications, and it's really one of the better ones until you realize that those sorts of fights can often be diffused by the victim before they get started; that doesn't mean the victim is at fault, but it means these are not the random, unpredictable, can-happen-to-anyone attacks that politicians want people to believe. If citizens are disabused of that notion, they are no longer afraid, and politicians lose some power.