Pirate metal is probably seen as "ironic," or maybe just too niche to deserve a top level genre (how many bands besides Alestorm are there?). Viking metal has a long history, though, given the Nordic roots of many extreme metal genres.

That being said, I don't think Viking Metal is too helpful of a genre anyway given the completely different sound of, say, Ensiferum vs Borknagar, or Tyr vs Windir.