
I've never been sure why this is. Maybe liberals feel a need to stake out each of their bumper sticker convictions separately, as they tend to be a coalition with different specific pet issues, while conservatives can use a single sticker (Bush/Cheney 04, for example) as a shorthand for their Gods Guns n' Gays agenda.
Interestingly, when I ask liberals about this, they claim it isn't true. They seem to believe that conservatives tart up the backs of their cars just as much.
When I ask conservatives, they claim it's true but only because they fear their cars will be vandalized by the mouth-breathing liberal hordes.
I don't think conservatives seriously believe this -- but you gotta feed that persecution complex somehow.