I would add - the comments section of typical left-leaning news sites have become absolutely fanatical if even one dissenting opinion is expressed. If you agree with 90% of a topic/idea and provide criticism of the other 10%, you are dismissed as a racist nazi and shunned from the group. Try it some time as an experiment, they swarm like flies to honey. With that type of environment you simply will never see disagreement, people have better things to do than shout at a wall. Since Slashdot has people with higher average IQ, and a marginally better moderation system, dissenting thought isn't punished and can be debated on it's merits (to a point).
There's also the simple fact that a percentage of people will naturally shift right as they get older. So, if slashdot's reader base has good retention without much "new blood" being injected into it, this change could manifest as a result.