Pron* won its obsenity cases due to the first amendment right, and we have to keep that only in certain places/away from children.
Why wouldn't they do the same for games? Especially for those games of an adult nature? Because games are specialty items. Sure wal-mart carries them..but there are full on Video game stores that would have to revamp the way they do things, if they can't even display Mature rated games.
Simple answer - the morality of our nation on balance. Despite the fact that we are a secular nation, we are made up of deeply religious people in general. More in some parts of the country than in others, and values and standards differ. But in general, (and VERY generally), Christians find obscenity more objectionable than violence.
Obscenity laws have been largely left to local community standards. A hotel manager in Cincinnati was sentenced to prison for allowing adult movies to be shown in the rooms of the hotels he managed; despite the fact that his hotel was part of a national chain and such movies were being shown in hotel rooms across the country. It was illegal there, and that was the community that set standards there.
So understanding that laws on obscenity are not based on presumed harm done to society, but simply on the desire of society to not permit it because it is morally objectionable, one can see that you can't really compare obscenity laws to laws against the depiction of violence. They're not the same thing to a deeply religious society; even when the logic just makes no sense at all.
This is an example of something I've talked about before. Laws do not have to be logical or based on truth or science. They only have to reflect the will of the voters and not infringe on civil liberties. In the case of violent video games, the majority of the voters don't really care, and restricting them does infringe on civil liberties. In the case of obscenity, the community cares very much, and the courts have found that community standards are sufficient to permit a minor (reasonable) infringement on what would otherwise be a strong 1st Amendment right.
We want our laws to be logical. They often are not, and in reality, there is nothing that requires them to be that way. This causes great consternation among the logical, which I totally get.