Because unlike movies, comics, and television, videogames are still a relatively new branch of media, and many people are not comfortable with them because they have little or no experience with them. They associate videogames with Super Mario Bros and Tetris, and so when they hear of or see a game that pushes or breaks the boundaries of what they've come to expect out of the medium, they are wary and react with hostility.
Coinciding with this, the newness of videogames as a medium, and the general lack of knowledge the past-generation world has of them, makes them a convenient and flexible scapegoat for psychological problems. Rather than blaming a perpetrator's upbringing or mental stability, it is far easier to remove the grief, guilt or shame from any one person and focus it on the gaming industry instead. People make the simple connection between guns in a game and guns in the real world and don't allow any proof (or lack thereof) to dissuade them from their conclusion that it is morally and mentally damaging.
Essentially, it is the public's lack of knowledge in videogames that makes them act with such desperate and flawed reason. In the words of Lovecraft, "The oldest and strongest emotion of mankind is fear, and the oldest and strongest kind of fear is fear of the unknown."
As previous generations die out and newer generations take their place, and as gaming becomes more accepted and understood by a greater percentage of the populace, we will likely see much less videogame-scapegoating.