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.