Because it's dramatic and flashy, Galt. Simple people like shiny things and loud noises. Same story is true of people flipping out over atomic bombs during WW2. They don't care that conventional bombing campaigns were orders of magnitude more deadly...those nukes are FLASHY and SENSATIONAL! Or people today who blame "the media" for every damned thing as if it were some sort of homogenized, monolithic thing controlled by a cabal operating out of a volcano lair.
We've got fstar literally claiming that "teh mediuh" orchestrated the big march the other day. With a straight face. He is that naive. The media is loud, always there, and sensational. So people freak out about the media reporting something instead of the thing itself. School shootings are dramatic, shocking, and perfect vehicles for emotional arguments and action. So people ignore the actual facts and fly off the handle. "We've got to do something!!!! OMG!"
Trying to apply statistics or even some measure of base rationality to any of this is completely pointless. Sheep are sheep. They have been well trained.