Because a giant burning cross on the MLK memorial is a pretty newsworthy thing. Giant burning anything, anywhere, is a pretty newsworthy thing. Vandalism in a cemetery is gross, but it happens in pretty much every cemetery on the planet once in a while. Unless it is some sort of extreme it's not news. Until it starts happening in some widespread and frequent fashion, then it becomes newsworthy. That the people committing the vandalism are probably acting out because they feel encouraged by "their guy" being elected president makes it more newsworthy.