I think this statement could be applied to anything though. Why do we enjoy a television show about working in an office more than we enjoy watching people actually work in one? Etc...
Perhaps because the idea of space travel is so inaccessible and a bit abstract to the average person? I wonder if more people would express interest if they became more cognizant of the concrete ways space travel benefits their daily life. Maybe someone should rent out some billboards and plaster them with some of these facts... http://www.spaceexplorationday.us/benefits/space-benefits-ma...
I think the better idea is to make the case to private companies-- they've already got some pretty good marketing corps that can convince the people AND we'd have an explosion in exploration. Look at how much SpaceX alone has contributing to interest in space.
They have been some moves towards it lately. I remember reading around a year ago about a company that was planning on a one way trip to Mars with some early colonization methods where they were asking for volunteers. Don't know if the company held together or the plan continued, but at least there is some interest in it.