it's because guns were readily available imvho.. but to think that someone hell bent on killing someone would simply opt not to kill because they couldn't get a gun is kind of silly imo..
plenty of other lethal options out there for any wanna be killer to use, outlaw one, and someone determined will simply find an alternative, of course, imvho..
The difference is between someone killing someone, and someone killing people. People get murdered all over the world, but where mass murders are involved, it seems to happen in places where guns can be easily acquired. It may indeed be true that guns don't kill people, people do. But it is also very arguable that people do not kill a dozen people in a movie theatre with a knife, but they can quite "happily" do it with their legally obtained automatic weapon. With the availability of guns, it also makes it a hell of a lot easier to kill people in the psychological sense. Stabbing people takes a lot of will power, indiscriminantly spraying people from a distance, not so much.
As to the criminal and home defense aspect that is always brougfght up with gun control, there is a reason there are so few armed home invasions in the like of the UK, because if a criminal wants a gun, he can easily get one, but they don't, because they know that the odds that there are guns in the house they're breaking into is next to none.