Because Warhammer has such a huge influence, and the term 'Dark Elf' is rarely used by other sources. The main conflict is Drow Elves, which are actually black, whereas the true Dark Elves, in Mythology, were Dwarves (Elves that dwelt in the underground).
It all depends which Fantsy source you grew up with. I've got 14 years of Warhammer behind me.