Why don't pyroxenites and peridotites appear in Streckeisen's QAPF diagram?
Why don't pyroxenites and peridotites appear on this diagram?


Because they do not have any quartz, feldspars, or felspathoids.

The same Wikipedia page also says this:


  QAPF diagrams are also not used if mafic minerals make up more than 90% of the rock composition (for example: peridotites and pyroxenites).


Instead, we use a different diagram, that has olivine, orthopyroxene, and clinopyroxene at the vertices. Like this one right here.


  Aren't they igneous rocks? 


Not necessarily, actually. But that's a completely different question.