Because it didn't have to happen.
There is something seriously wrong in a society where people's lives are potentially at risk when they go to school, attend a popular movie, or go Christmas shopping, because the gun lobby and some misinformed enthusiasts of the Constitution insist it is the right of Americans to to own guns. It is not.
The Second Amendment reads "A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed." Period. The end.
An individual with a gun or a rifle is not a militia, and a country that cannot protect its citizens from the kind of senseless violence that happened today is neither secure nor free. It is a state held hostage to fear.
It should be the right of every American to be able to go to school, see a movie, or go Christmas shopping and not be murdered in the course of pursuing everyday life, liberty and happiness. And for those in the "guns don't kill people, people kill people" camp, sure- okay. Let's assume that's true. But people armed with guns kill people much easier and in greater numbers than they do using any other method.
And the fact is, it's going to be easier to control the distribution and availability of guns than it is to anticipate the actions of an insane person bent on murdering as many people as possible. And since we can't take the guns away from just the crazy people, the American way to do this- the right way to do this- is to take the guns away from everyone.
And the time to do it was a long time ago. If you don't think that's fair, that that is somehow un-American, tell that to one of the parents whose child was killed today. Please explain to them how it is more important that America be able to arm itself than it is to expect that its children will return home safely after a day at elementary school.
My deepest condolences to the families of the victims, and for the community of Newtown.