The reason is because you can't pulverize a star. It's immaterial for thematic reasons, just like Hope (I'd imagine) is immaterial for thematic reasons. You can destroy a shield with a explosion or steal someone's Hourglass, but I'd like to see you destroy a star (just because they can explode, doesn't mean you can make one explode). To compensate for the immaterial, the upkeep is 2
per turn, a somewhat sizeable amount. The cost would be less if it weren't immaterial, but it doesn't make sense (to me at least) for it to be effected by PC.