Well, the main reason is that politicians are not interested in actually solving problems they're only interested in getting (re)elected so they propose solutions which are popular with the largest group of people ... Violent videogames make a good scapegoat because (of people who can and do vote) few people play violent videogames.
At the same time, I think there is some reason to be worried about the influence of violent videogames; in particular the influence of violent videogames on younger gamers. It is believed that a vividly imagined event can be as powerful as actually being part of that event, and this is used in several types of therapy (and sports psychology) in order to get results out of people. As videogames become more realistic (and potentially immersive) what kind of impact does performing violent acts really have on a person? I'm not saying these games should be banned, but I don't think it is wrong for people to question their impact.