Because Kaaba is a place of god, not a museum. It is probably the most important place on earth for Muslims. In Islam, one of the 5 pillars (obligations) is to do hajj (pilgrimage to mecca/Kaaba). As a muslim, when and if you can make your hajj, it is a very spirutal and emotional thing. To open up to the public wouldn't be right because of it's religious meaning.