User blog comment:Godzillavkk/New Characters for Star Craft 3/@comment-3581786-20191014163857/@comment-31277203-20191021012245

You don't know that for the primal zerg. If you did the Zerus missions and THEN went to Kaldir, Dehaka would ask Kerrigan why he couldn't assimilate protoss essence. Kerrigan explained that outside Zerus, there were things he didn't understand. And Dehaka did say that upon assimilating terran essence, he'd understand them better.

To me, I don't think the primal zerg aren't interested in galactic conquest, it seems more like they never evolved creatures capable of deep space travel and therefore were never able to leave Zerus.